[Adam Lear’s answer is correct][1], but there is one other possible (and in a way, more innocent) reason for extremely long suspensions: The Super User moderators at some point started to merge all spam accounts into a single user (this has several interesting implications for spamfighting) instead of outright deleting them. [The target user][2], into which 487 users have been merged to this date, is currently suspended until 2025. There’s obviously no reason to ever have this “user” not be suspended.

I don't know if other site moderators have started doing this as well.


  [1]: http://meta.stackexchange.com/a/134412/243319
  [2]: http://superuser.com/users/43131/deposit