It’s basically one of the rare cases of almost complete anarchy which is actually permitted by the authorities, and it just so happens to be at the same time as Guy Fawkes celebrations. The whole thing is largely comprised of rivalry between local societies, which results in a big explosive showdown, and is really aimed solely at local residents. Unfortunately for them (and the NHS...) a lot of non-local people try to visit because it’s fun to watch, and gets the adrenaline going.
The rival event for family-friendly stuff (ie. not throwing chunks of burning material at strangers...) is Brockham Bonfire in Surrey, which usually attracts 18-20,000 people just in the Village Green and is much safer (with infinitely better fireworks, in fact the largest annual UK display) and there are also other big ones in a lot of villages in Surrey and Sussex - I’ve attended several over the years. They don’t tend to crop up on here as they don’t impact railways, but they do have extremely major road impacts, to the point that if you live in the Weald, you may as well add half an hour to any journey time during the evenings of the applicable weekend.
There are also the Battersea Fireworks, which do affect trains, but there are several stations in the vicinity which help spread the load, with lots of extra staff, and many more trains than the Lewes area routes. Even so, part of Battersea Park station can’t cope, so it is partly closed for the event - with passengers redirected to Clapham Junction and Queenstown Road.
(All this sounds like I don’t like fireworks and bonfires - I do - but I understand local sensitivities!)