There is not much clarification due to there not being a reason as to why they closed your account. Are you sure they didn't send you other emails or notifications stating the exact reason? I'll have to assume that the country or state you live in is excluded for legal reasons, or you have somehow violated their terms of agreement, or you are underage. Mind telling what country you are from?
They at least don't suspect you from illegal activity, otherwise they wouldn't have allowed you to withdraw anything.
Most likely this would be a country issue or any other means and as you said its not being stated on what is the actual reason why you do really need to be mandated to pull out your funds? Its a little bit confusing or do mind boggling why such rush decision been made for a user. The good thing on here is that they do still able make you some notifications on that matter so that you can able to withdraw.
I do really like to make clarifications with this one, I might email them later on towards this matter if op wont able to do such thing.