I've heard similar stories with Poloniex actually and hope yours doesn't turn messy. They seem to just freeze accounts arbitrarily and don't communcate well with the users once they've frozen it. You don't seem to have done anything suspicious, but like said above, it could be just because of an association with a suspicious account. Just being connected with one is enough to be blamed, so it's probably not a good idea to keep a large balance in the same account that does transactions. A better solution is to keep the majority in an offline wallet and then transact as necessary with the rest though an exchange.