I think Shweta and Cez, since they were both very very young when KZK started, fell for each other. She spent most of her time with him on the sets doing romance. So for a 20 yr old to fall for a handsome hunk like Cez should not be surprising right!! And Shweta is so pretty any guy would fall for her!! Married or not. Anyways later she probably realized that she is married and all, and Cez probably wanted her for good (which meant she would have to leave her hubby and all)...she realized what she was doing and they had a fight...the end of the story!! My hypothesis!! I have no proof!! Just thinking what might have happened!!
SIGH!!! Famous couples always part ways after a fight or end up married!! SRK and Kajol, the only exception (but Ajay doesn't like SRK...haa haa).