Sita passing the fire test, and the intervention of the gods, did not reassure Rama, for long, of Sita’s fidelity and soon his doubts and jealousy re-surfaced and he was treating her like shit again
things came to a head when she announced she was pregnant
dismissing the possibility that he was the father, Rama took this as proof that she and Ravanna had been screwing one another during her ‘captivity’ and so threw the poor cow out
