Reason is that Buaji said that Garima married Shashi 17 years ago and according to her Garima was really devoted to shashi and payal .
Arnav's parents committed suicide 14 years ago. That means that when his parents committed suicide Garima was already married for 3 years and she seems quite devoted to Shashi unless she is also pretending like Shyam though I don't think so.
In flashback the mother said that she had seen her husband and 'that" woman together. She must have seen them together on the same day because if she had seen them earlier she would not have chosen to commit suicide on the day of her daughter's wedding.
Garima was referring to a lie that she told while talking to Shashi. Shashi seemed to know about that. It seems like some big MU .
It seems that Garima told some big lie that led to an MU. but why? was she under some pressure and by whom? maybe that "whom" is the main villian.
just my thoughts.