Not that I'd be surprised if it were, but did you perhaps try to access that account from an unusual location or device?

I ask because I had the same thing happen to a little-used Twitter account of mine. Killing time while on vacation in a foreign company, I attempted to log in, and failed. Not sure if the failure was my fault or Twitter's. Nonetheless the outcome was the same -- the account was suspended for "breaking rules", despite not posting anything. An appeal failed, and I lost the account permanently.