Now that Zak has been dead OVER two years and we know Lee and Kara hadn't seen each other since the funeral....Lee is now faced with all of those old feelings and I don't think he realized just HOW MUCH he felt for Kara until he had to deal with her being with another guy.
For Kara I believe that she has been somewhat risque in her dealings with the opposite sex and she doesn't always think through just who she is going with .... Baltar cut in on Lee and Kara dancing....it is a pretty fast moment in the episode ...so if you are not looking for it...you may have missed it. Lee seemed somewhat *rejected* and left... while Kara was dancing and drinking the night away...and when she stopped to look....NO LEE so I think her own feelings of insecurity with Lee all came rushing too along with the alcohol....and as many woman who are insecure do...she took the *love* that was in front of her. BUT when it mattered most her true feelings YELLED

out
So that brings us to Lee figuring out that *something* happened with Baltar...... then when he confronted her...he was angry BUT he didn't KNOW for sure just what happened...so the more he talked to Kara and she wasn't denying anything...well the more that green-eyed monster was overtaking him.....
Then once he was SURE he lashed out at Kara and was basically calling her a *not so nice name* and Kara being Kara DECKED him...well at this point I don't blame Lee for decking her right back....if a woman hits a man like a man then expect to be hit back. At that point I believe they were both so wrapped up in their own INNER DEMONS...Lee...having ALL of his feelings rushing forth all at once...and Kara...feeling like a total schmuck because she really didn't *want* to sleep with Baltar AND she was feeling a tremendous amount of guilt and alot of her own feelings are still so intermixed...plus Kara IS dealing with the fact that she was engaged to his brother...and now she is finding that she has some REALLY strong feelings for Lee...that has got to mess with her...