Could it not simply be that any online group that's devoted to supporting a person or a cause (not just political, but fans of bands or actors, or ethics in gaming journalism, of course) is going to have a certain subgroup who are particularly nasty and vindictive trolls?
Bernie Sanders has a considerably larger and more active online following, apparently, than do any of the other candidates, so the group of Bernie trolls and sealions is almost inevitably going to be larger than that of Biden or Warren trolls.
I suspect there may be other factors at work, too; it would surprise me if the flying monkeys from 8-chan didn't think it was a jolly jape to upset the normies and libs by acting like particularly obnoxious Bernie supporters.
I'm no particular fan of Bernie Sanders, from what I've read about him, though I may possibly be prejudiced by his apparent similarity, in many ways, to Jeremy Corbyn, but I do try to be fair.
Corbyn certainly had a large, and very active, body of supporters, some of whom could turn very nasty indeed online to anyone who was less than enthusiastic about him, but the nasty ones were only a very small minority of his supporters overall, and I suspect the same may be true of the Sanders campaign.