The answer to why there are numerous snake groves or sarpa kavu in Kerala is found in the book 'Kerala Ulpathi' by C.K.Menon. Sarpam means snake and Kerala is famous for snake groves. The history of these ancient natural temples stretch back to the origin of the land of present day Kerala.
The district of Malabar along with Kochi, Travancore South and a part of North Kanara, extending from Cape Comorin to Gokarna, distance of 640 miles north to south, was reclaimed from the sea by Parshuram Avatar of Vishnu and colonized by him.
The aim of Parshuram Avatar was to annihilate the evil Kshatriya Kings who were practicing adharma on earth. He carried out his object by destroying the warrior chiefs 21 times. This sin of killing was too much for Parashuram. He therefore retired to Gokarna in North Kanara (present day Uttara Kannada, Udupi and Dakshina Kannada)
Parashuram invoked Varuna, the god of waters, to give him some land. Varuna accordingly went back a few miles, and 646 miles of land, called Kerala came into existence.
Various gods and goddesses from Hindu Pantheon were then fixed in 108 places and learned people arrived to settle in the newly formed place.
But the place was infested with poisonous serpents and therefore the new settlers ran away.
Parashuram again invited several people to the new land and divided the new land into sixty four colonies. He advised the new settlers to regard snakes as their household gods, and to reserve special places in the land for them. He ordained that, in every household compound, a quarter of the area should be reserved for the snakes. The site should be cool and near pond. Trees should be planted. The land allotted to the snakes should not be tilled. There should be minimum human interaction. After this the newly found land became free from the fear of snakes.
This is the reason why there are several snake groves in Kerala. The famous among them are Mannarasala, Vettikadu, Pambummekkattu Mana, Nagampoozhi Mana, Pampady Nagaraja Temple etc.