I'm guessing you're more used to Reddit because you talk about sub-forum, which is not a term used on Mumsnet. I'm also guessing you've never spent any time on any other part of Mumsnet. What you are describing is not behaviour you only see here, it's standard behaviour across the whole site.
Poster A says something obviously silly. Poster B replies saying as much. Posters C, D, E and F were all typing at the same time and reply in similar terms a few seconds later. The thread moves on very fast, but ten minutes in Poster G reads the OP and replies saying much the same as Posters B to F. Give it half an hour and Poster H will do the same thing. Ten years later Poster Z will do it too, even though the thread is by this time nearly 1000 posts long and everyone originally involved is long gone.
The original subject of the thread could be absolutely anything, e.g. 'Nobody really likes fruit, do they?', 'All Americans are thick', 'I'm going to leave my 13yo son on his own for a week while I go on holiday with my new boyfriend, that's OK, isn't it?', 'I do X with/for my child because I am a good responsible parent and I don't understand why other people do different things', 'Dogs are better than cats'.
Welcome to Mumsnet.