They did it on accounts that had suspisious behavior (thats what I heard most of the time).

When I called them, I was talking to a friendly person, and they explained how they randomly do it to accounts for security purposes. Basically, when the account is limited/locked, they make you send in a copy of your drivers licensed, a signed agreement to their TOS with a notary signature, and confirm your address.

I guess this was illegal for them to do.