Lord Ram after being established as the king of Ayodhya was advised to perform sraaddha(ancestoral ceremony) for his father Dasharatha at the holy place of Gaya. Lord Rama accompanied by Sita and brother Laxman came to Gaya. As per the instructions of the sage who was going to perform sraaddha, Lord Ram & Laxman went out to fetch the necessary ingredients. As auspicious time for the sraaddha was passing by and Lord Ram- Laxman were nowhere in sight, the sage asked Sita devi to do the pinda dana(offerings) herself. She was asked to make balls of sand (instead of grains) from the river Phalguni (Phalgu river).
Sita devi’s performance of the pinda daan(offerings) bore fruits when Dasharatha Maharaj’s hand appeared from underground to accept the offerings made by her. Later when Ram & Laxman arrived with the ingredients, Sita informed them of the entire story. Lord Ram wanted some proof for this incident. Seeing a cow standing nearby, Sita devi asked her to give witness. She also asked the river Phalguni, a Banyan tree and a Thulasi plant and a brahmin guy, who were watching that, to bear witness.
When cow & the river did not give witness, sita devi became upset & cursed both of them. Cow was cursed to have her mouth always unclean due to continuous drool of saliva and also was cursed to have her sacred Lakshmi amsam at her back. And river was cursed to be waterless on its river bed. (River Phalguni thus became Phalgu which also means False River). She asked Thulasi plant for witness, but it also failed to give witness. So Sita cursed her to be placed in a backyard instead of being in the front of the house. In the same way, the brahmin guy also did not give witness and so she cursed him to become a Sandi Brahmin, who would always be considered to be a dead man in the 13 days ceremony of the dead.
The banyan tree, however, gave witness. Sita devi became pleased & offered it the blessings of becoming immortal & never shed its leaves throughout the year.This tree is today called as Akshay Vat(immortal banyan).