It's having babies.
My PILs were lovely until I had a child with DH. I have DC from a previous relationship and all was well. It was quite a few years into the marriage before we had our own DC, and all was well.
Once the baby was here, they just collectively lost their shit for a while. They couldn't get their heads around us doing things differently from how they did it and took everything as an unspoken criticism. I think they blamed me, because it was easier than accepting that their son was his own person who thought differently from them.
There was also an issue with SIL, who is consumed with jealousy and resentment of me because I had her preferred sex, and because she can't cow me.
However on saying that, things are calming down now DC are getting older and they're becoming used to our boundaries. They aren't horrible people at all, I'm very fond of them, but very enmeshed and SIL stirs the pot.
It helped that it wasn't my first rodeo either, I had the pleasant breezy "this is how it's going to be and I'm going to ignore any nonsense" down pat, which meant that the incidents that could have blown up into a family row didn't, because I just didn't acknowledge or reward bad behaviour with attention.
I'm also no pushover and I don't give a shit what people think of me
If I'd been a first time mum though, it would have been dreadful and we probably wouldn't have been speaking now.
As it was, the switch in their behaviour was stunning. It really was overnight. And these aren't bad people. I posted about it on here at the time and many people shared their own stories,which were ten times worse.
Honestly it's when the babies arrive. I don't know why, but it makes certain PILS lose their shit