I know what people are thinking, but it doesn't necessarily have to be anything untoward.
She may have been reluctant to "settle down" in the first place, and a good party reminded her of what she was missing. Or something along those lines.
Or maybe the bachelorette party made her realize that simple pleasures were meaningless compared to her deep love for her partner, and she couldn't stand being single a moment longer, and so they ran off and eloped that very night.
This happened to a guy I knew but the other way around. He turned up late to the bucks party, started crying and saying he wasn't ready for marriage and ended up cancelling everything and paying the bride-not-to-be's parents back for everything.
I think they just rushed into it and he realised at that moment.
Bachelorette parties are about being untoward, so that's why people are leaning into it.
You could still be right though. It may also be that her fiancee was controlling or shitty even if she didn't do anything "out of bounds," and that's what blew the relationship up.
It could have also been an abusive relationship being forced by her parents and family and the Bachelorette party was the cover for her extraction from that situation.
Tbch I'm wondering if that's the secret girl code reason why the bachelorette party exists in the first place, so that the bride to be has an opportunity to veto the marriage and escape the family that was pressuring her into it.
I don't dont think its generally that deep. Most people like to party, and the "marriage send off" is generally considered a wildly bacchanal one, if not outright hedonistic, at least for everyone else attending.
The intended lack of inhibitions probably does cause issues to bubble up, and it honestly likely ends relationships that probably should be ended, even if the people involved won't acknowledge that sober.
I also wouldn't be surprised of it is used as an escape hatch by some, perhaps by "unintentional" behavior that can be blamed on the party itself.
My friend's fiance was like that. I thought she should've canceled the wedding after the way he acted during her bachelorette party. He insisted it be a joint event with the bachelor party and then picked fights over everything she did. He called her a slut in front of everyone (meanwhile, she found condoms - which they didn't use - in his bag a few days earlier) and was generally a complete asshat. I would've been thrilled to see that notification.