Sonny had Ava under surveillance. The day of the accident--the guy was snapping pictures. Kristina threatened to bring up all of Ava's crimes if she was forced to take the stand. Kristina had her back to the window. She went to walk past her and Ava placed her hand on her arm and Kristina pulled away, tripped over Cate's duffle bag and the momentum caused her to crash thru the window.
But Now here's where it gets wacky. TPTB replayed Kristina going out of that window like 4/5 times. The pictures show Ava with her hand draped over Kristina's shoulder. It didn't even look natural. Imagine if you were place your arm on someone to dance or you are about to pull them into a hug. But Anna, Michael, Sonny etc all believe that the pictures prove Ava was the aggressor. Kristina made Michael show her the pictures. Making things worse is Ava lied which made her look guilty. She told the cops that Kristina lunged at her and that she tripped over a shoe. Why she did all of that, when the bag is way bigger is beyond me.
Here is a screengrab of how Ava actually touched her arm
Notice the position of her hand in the picture. That never happened.