Paul Graham does something similar. (He's blocked me and we've never interacted.) He said somewhere that he blocks people when he sees them behaving badly, or something like that, and that in the past when he has randomly stumbled across somebody he had previously blocked and taken a second look at their tweets he's felt only vindication.

Now, look, Graham blocked me on these grounds, so I'm not thrilled to say that I think he's completely right about this. I block people I haven't interacted with all the time. Not everybody on twitter is a good actor there, even if they're nice people in other contexts, and it makes life easier to kick some of those people to the curb before they're even aware you exist.

I probably said something tendentious or snarky or unfair or maybe even mean and I might have blocked somebody for the same thing, were the shoe on the other foot.