Most older forums had an element of self-selection... people don't hang out where they're not wanted. But with Reddit, it's the only forum left (for any number of broad and narrow topics) so you're either there or nowhere. This forces to some extent people who would gravitate away from each other, and personalities go overboard. There's more need of a judicial process there, than there would be elsewhere. And that was before everything became politically polarized. Now that you could be perfectly happy talking to someone about X, you still end up hating their guts because they love/hate Trump/Obama and it slips out (over a long enough timespan).

People do not scale.