> That's really not how they're treated.
It really is, in my experience, at least in the rare cases where the reason for a mob downvote isn't clear.
> I don't present every stranger who bumped me on the bus...
But then, you have a single, specific focus for the "downvote": Some single person, who's apparently having a bad day. The cause is obvious, won't be improved by questions, etc. It's a good example of a case where the rule-of-thumb applies. Further, your "questionnaire" could be a raised eyebrow to the person next to you, who might look up, shrug, and turn away.
The whole point is to see if you're being the asshole, or if it's the other guy being unreasonable.
Now, imagine that everyone on the bus is self-selected for a trait besides wanting to travel somewhere. Say, they're tech or business people, congregating for the chance to talk about things that interest "hackers". They're there specifically to talk and discuss. A driveby downvote doesn't aid that goal. An explanation of why the commenter is being unreasonable does, even when it comes as speculation from someone who didn't downvote them.