Here’s what Mike Barr had to say in Back Issue #73 (2014) about what happened to Catwoman:
My first marching orders from Denny O’Neil were: A) Make Catwoman a villain again, and: B) Erase from her memory any knowledge of Batman’s secret identity. We decided a story using the Joker would be the best way to do this, as having Batman and Catwoman working together would drive him crazy (or crazier). And, since the Joker and Catwoman first appeared in Batman #1, it was also a very subtle tip to the readers that we were starting all over again.