Brain damage from the freezing process? Did seeing your friends and family get nuked make you snap?

The thing about multiple personality disorder is that it usually occurs as the result of severe psychological trauma, usually in childhood. That, combined with the fact that they probably wouldn't freeze someone they knew was crazy when there were so many other people they could freeze means that either you have to have skills they couldn't risk losing, or you had to have developed it after they ran the psych evaluation.

If you can come up with a reason, though, that's fine.