Tumblr account terminated?

That's strange. Sorry I'm only just seeing this now! Did you ever find out what caused this to happen?

One possibility, I suppose, is that you might have the same external IP address as a tumblr user who got banned for life. Even though external IP addresses are supposed to be unique, there's really only so many of them and sometimes freak things like that happen. However, they also change from time to time. You might find (if you haven't already) that if you try to create an account again, you will have no problems.

(BTW I've apparently shared external IP addresses with people from Japan and New Zealand before. I live in England.)

It could have been something else of course, but like I said, it's worth trying again if you haven't yet. Whatever it was, it could have easily just been an in-the-moment freak thing that will never happen again.