The email I got said it was the character. From the email: "If your game account is inactive for over 90 days and you reactivate your account you will need to log back in to any characters affected by this policy to move them out of unreserved name status."
I had like 14 characters or so, but only 2 or 3 I really cared about their names. Black Prophet was the main one. With City of Villains coming, that name would be gone real fast. And I was never so hardcore into it that I got a character over level 13. I'd get to level 11 or so, and thats about where it took around 2-3 days of playing to gain a level when it first came out, so I would start a new character, get him to level 11, start a new one, etc. Now its a lot easier to gain a level once you get around that level. You can gain one around every 3 hours of play or so, which pretty much amounts to getting a good group and doing some missions that are 1 or two levels above you.
And for those curious, the character I play most is White Pulsar on Guardian. Level 13 Energy Blaster.