If you look at the top questions on the subreddit, there isn't a great deal of correlation between subject matter and the number of responses. There are, as you say, people that feel the need to issue contentious or low-quality comments to certain subjects, but before that happens they generally need to see the question, and that's a matter of ranking. The Cold War hysteria thread is a great example: it's a question which attracted a lot of attention very quickly; so quickly that it hit r/all, which opened it up to a wider audience and attracted more attention. The more people see it, the greater probability somebody is going to post a response to it. In that specific question's case, the wider attention brought in a lot of redditors who aren't familiar with AskHistorian's rules, which caused a lot of low-quality comments, which were spiked by the mods, which can trigger a feedback response as more people ask why the comments are getting removed (or arguing about why), and eventually AutoMod gets turned on - which is why you can get a very popular question that looks like an absolute graveyard of removed comments.

However, the popularity doesn't correlate directly with the subject. Questions related to contemporary politics and soapboxing usually get removed by the mods before they get many comments, if they're even approved in the first place, as that's against the rules. Current events can often cause a spate of related questions in a brief period of time - there were a large number of questions related to Confederate monuments during and after the events in Charleston, SC earlier this year however - but those questions don't always or even reliably generate a lot of bad comments.

Moderation isn't usually difficult - removed comments usually are poorly cited or not cited, include anecdotes or links to unreputable sources (basing an answer on a Wikipedia article is not sufficient), express personal opinions, etc. Maybe surprisingly it's not "big questions" that attract a lot of attention (and the consequent flood of low-quality comments), but little questions which people have a surprising amount of interest in - two of the top questions on AH include edible dandelions and bread.