might be Kunal goes to diwaans to begin with , over hears surekha and Abehy's conversation and Sidhi's consent for the marriage.. obviously it would piss him off .. jealously, anger etc ..
also he might end up thinking , Sidhi said yes so that getting AC custody would becomes much more easier (since these days Kunal's brain is working in reverse order ) so his insecuritie and his fears will again creep up
He might decide he has to stop this marriage from happening by hook or crook and
then comes the pre-cap scene at the office where he might have planned in such a way that abhey would come exactly at the time when he is close to Sidhi and obviously seeing his would be close to another man any one would reconsider his decision and Kunal plan successful from his POV..
Becahri sidhi unaware of all this ...