I find that AIBU is often frequented by people who seem to waiting around rubbing their hands together in anticipation that they will get to trip someone up for sport. I have seem some absolutely upsetting cases where the OP has a really legitimate issue and people have absolutely torn into them in a way that I just thought wasn't normal and didn't follow the natural logical response that most functioning adults would have had. One thread in particular nearly made me cry because of the absolute blatant unkindness. It felt so absurd that it was almost like something from a bad dream.
The thing is, often the response is that people cant stand being told they are wrong or that they can't handle debate. My issue the way some people seem to really extrapolate wild scenarios and assumptions from the OP, and almost deliberately misunderstand the post.
Another thing is that mumsnet is populated by mostly mums. I don't think it would be a huge leap to suggest that many mums are pregnant, post partum, sleep deprived and dealing with the immense emotional vulnerabilities that come with that.
I remember being on a similar board to mumsnet that also had an AIBU and the vibe was the same. If you worded your OP just right and made sure to caveat everything and front-foot every accusation that might be hurled at you, you would get support. But if you worded things clumsily or didn't set the tone appropriately, you got it. I remember I had a traumatic birth with my first and looking back, had PTSD. I actually turned into another person and really needed therapy in retrospect. I posted something and was torn apart. I remember not being able to speak for the rest of the evening and taking the words so deeply to heart that I started spiralling and am surprised it didn't kick start some kind of episode.
Looking back I see that I was incredibly vulnerable at that time and left myself wide open. I stopped opening up for a long while after the AIBU incident when I should have been trying to get counselling. I stopped reaching out for help. It makes me want to cry about it a little bit, when I look back, because I was a new mum just desperate for connection, who felt like the bottom had fallen out of my world due to the trauma and the responses I got went beyond robust debate.
Not everyone knows they are very vulnerable or that they can't handle AIBU. Not everyone knows what its like or that you do need to prepare yourself to go into it. There is always a person behind the screen and you dont know what damage your words might do.
I like a debate, I am happy to tell people when I think they are wrong but I try to do things without harshness, as I do in real life. I learn a lot from mumsnet but I put my hard hat and approach AIBU with my special AIBU suit. I know the landscape now.