Now, it's possible that someone forged this person's address, but I find
that implausible, nor does it excuse them replying to letter after letter
cursing out all and sundry.

I've noticed that there are some people on this list who are from the same server. A glitch in the server, or perhaps the server-owner's adding new newsgroups or messag bases could've taken mail meant for that person and copied it to other persons' accounts.