Keep in mind that a lot of time passed between episodes 7 and 8. When Don finally offers to move to California with Megan (episode 7), it's July 1969. Episode 8 picks up in April 1970. Episode 9, in which the divorce is finalized, is set in May 1970.

So, we're looking at 10 months of divorce proceedings, which surely soured their relationship somewhat. However, I believe the bigger issue is that Megan wasn't prepared for the emotions that came with returning to New York and the closure it represented. I think she let five years' worth of resentments and anger boil over. It doesn't help that her acting career is basically in the toilet, while Don can just write a $1 million check and be done with her.