That's disturbing since a hellbanning is a much more vile punishment than a simple ban. Why is it doled out so casually?

It's probably a case of an anti-spam algorithm gone wrong. At least, that's what I remember hearing about a previous incident.

No, it's almost always done directly by a human if your account itself is hellbanned.

The only case I'm aware of where pg has commented indicated that a person posted a link that was being spammed on HN in one of their comments and got banned accidentally.