There are many folk tales and theories regarding the migration of Iyers to Kerala. Some of them are: -
1.The present state of Kerala was a part of Tamil Nadu till eleventh centaury A.D. There were Brahmins in all parts of Tamil Nadu and there was no difference between them as all of them were speaking a highly sanskritised version of Tamil. There was a Muslim invasion at that time and the areas around Madurai, Kanjeevaram and Thanjavur came under the influence of Muslim hoards from North. Kerala being mountainous and inaccessible got separated itself from other areas and started developing a culture of its own as well as its own language. This continued for four centauries till the kings of Kerala (Venad Kings) conquered parts of Tamil Nadu especially the Pandya kingdom. This opened up the migration again and Brahmins of Pandya kingdom went to Kerala through the Aruvamozhi pass near Trivandrum. But by that time the Brahmins of Kerala had developed a separate identity of their own. And called themselves as Namboodiris. They were much richer and ruled over small parts of Kerala. The Tamil Brahmins who migrated were not rich but were essentially businessmen and spread all over Kerala. With the encouragement of local kings they were able to establish agraharams(Brahmin streets) in Trivandrum, Kollam, Kayamkulam, Kochi, Trichur and Palghat. They developed good relations with rich merchants who were Muslims, Jews and Konkanis. They carried the merchandise to interior parts of Kerala and sold them to rich Nayar and Namboothiri houses. IN each of the above places they established community homes (Samooha Madam) to stay as well as to meet. Wisher a Dutch priest living in Kerala in the seventeenth century in his book "Letters from Malabar" has indicated that the kings helped these Pandi Brahmins in their business efforts. They were also given free food and they were given exemption from customs duty. While this was going on large number of Brahmins also migrated from Tanjavur through the Palakkad pass and were helped by the Palakkad king in places like Sekaripuram. (Named in memory of a famous Palakkad king). But this group was much poorer and earned their livelihood by manual work initially. (K.V.Krishna Iyer, 1962, The Brahmins of Kerala, Mathrubhumi).
2.There was big famine in Tanjavur District of Tamil Nadu after the collapse of Vijayanagar empire in the year 1565.Since this area was under Muslim rulers at that time, the poorest of the Brahmins chose to migrate to places where free food was available and where there was no fear of forced conversions. A similar famine in the eighteenth centaury is supposed to have driven another horde of iyers to the former Travancore state. They chose to migrate to Kerala through the Palakkad Pass of Western Ghats. The Palakkad king (or according to some the Raja of Kollengode) welcomed them and gave them eighteen different pieces of land to settle down. Some of this land was taken as Kanam i.e. 12 year lease. These are the original Palakkad villages or Grammams.Dr.E.S.Narayanan is of the opinion that the first such settlement was Sekaripuram named after Raja Sekari Verma. And the second such settlement was Chokkanatha Puram. He feels that people from Chozha Desa settled in Sekaripuram and those from pandya desa) near Madurai) settled in Chokkanathapuram. He cites as support the establishment of temple of Meenakshi in Chokkanathapuram in support of this theory.( DR.E.S.Narayanan, Brahmins of India, P. hd Thesis submitted in Arizona university).
3.The migration started with Uddhanda Sastrigal to the court of Zamorin to demonstrate his Pandithya. There are several folk tales regarding his entry. The most popular one says that at the first village in Kerala he visited a temple and while praying told that the lion from Tamil Nadu is coming to conquer the Elephants of Kerala. The Marar who was playing drums at the temple retorted back saying the elephants of Kerala is not afraid of this lion. He is supposed to have been appointed by the Zamorin as the Asthana Vidwan in his court, it is told that he used to keep a parrot with him and first all the scholars who wanted to argue with him have to defeat the parrot in order to show their suitability to face him. Kakaseri Bhatathiri who was in his teens went along with a cat that crossed his path and then set his cat to argue against Uddanda's Parrot. This silenced the parrot and then later The Bhattathiri defeated Uddanda. There is a possibility that the Iyer migration took place during this time. There is also a story in Which Uddanda Sastrigal was invited to attend the Sraddha (ceremony for ancestors) by a great Namboodiri family. This again shows the acceptance of the Namboodiris of the Iyers as equal. There was also another Tamil Brahmin in the courts of the Travancore king Pooradam Thirunal Devanarayanan (1560-1690) called Neelakanda Deekshithar. He was followed by Ramaiyan who was minister of Marthanda Verma a king of Travancore and Swaminatha Pattar who was a minister in Zamorins court.
4.There is a story that The Edathilachan of Palakkad (The king of Palakkad) (there are some who feel that it is not Edithalachan but the Raja of Kollengode) was banished by the namboodiri Brahmins (bhrasht) and he brought the learned Brahmins from Tanjore and gave them important positions in his court because they performed prayschitta for the sins he did and also settled landed property on them. Kombi Achan of Palakkad Edithalachan family established the Kalpathi temple in the year 1425 A.D. Laksmi Ammal a rich widow of those times is said to have contributed large property for establishment of the temple as well as conduct of Rathothsava. One of the families that adorned his court is the Karyakkars Of Kalpathy. Even today on the day of Rathothsava of Kalpathy, the procession first goes to the Edathilachans place as well as that of Karyakkars. It is interesting to note that one Lakshmy ammal is supposed to have given money to perform the Rathothsava.
5.There is also another story regarding the refusal of Namboothiri Brahmins to cremate the mother of Adi Sankaracharya and then it is believed that he in anger brought the Learned Brahmins from Tanjore. These people on their way back were requested to settle down at Palakkad by the Palakkad king.
6.It is a fact that Kerala does not grow cotton and the entire requirements of their cloth used to come from TamilNadu. The iyers were itinerant merchants who use to bring the ready to wear clothes for sales in kerala. They used to do this during the sowing season just after Karkidagam and then come again during September to collect their dues. Some of these people brought their relatives to settle down in this new land.
7.During the reign of SwathiThirunal in Travancore he gave lot of encouragement to musicians to migrate to Kerala. There are historical records that this happened. This is definitely a migration from Tanjore to Kerala.
8.The nearest town of importance to the areas of Tirunelveli and Ramanathapuram districts of Tamil Nadu was Trivandrum. There is a recorded migration from these districts to Travancore Kingdom in search of jobs. This is a dynamic affair and is still continuing.
9.Kerala Brahmana Sabha has recently published a book entitled " Tamil Brahmins of Kerala". In this book the following points have been made about this aspect:-
a. A Raja of Palghat fell in love with a tribal girl, and decided to marry her, which was not approved by Nambhoothiri Brahmins of his Kingdom. He expelled all Namboothiris from his kingdom and brought Tamil Brahmins. He also gave them land to settle down (see point 4 above). The book also mentions that the migration was not via Coimbatore to Palakkad but from Pollachi to Chithur.
b. Sekhari puram village was gifted to them by Raja Sekhara varma of Kollengode.(see Point 2 above)
c. The rulers of Alathur brought Brahmins from Tanjavur and Thirupathi and gave them the village
d. Thunchathu Ezhuttahachan happened to visit Tamil Nadu and was impressed by the Tamil Brahmins and was instrumental and getting them settled in 3 villages in Chittur.(See write up about Chittur-Ezhttachan's Samadhi is situated in one of these villages).
e. Shakthan Thampuran of Cochin brought the Pandi Brahmins from Madurai to do cloth business in Cochin state.
F. They migrated from near Madurai during the rule of Mara Varman (14th century )- few via Dindukkal and Madurai and few via Tirunelveli and Trivandrum.