Demanding your seat back
Or maybe she felt intimidated into switching and after some thought decided to restore the balance. Some women aren't basically assertive and some men don't realise how they come across .
I have no sympathy for the guys if they wanted to sit together then they should have booked that way. Seat poachers are a pain and I've had guys think they can intimidate me into swooping either by arguing or dismissively telling me I can sit in theirs ......in that last scenario not a snowballs chance in hell would I reward such crassness with my seat!