You probably need to do kyc (send some documents)
I've never heard of Coinbase doing that. You're either verified or you're not which to me is vastly more preferable than being kept in no man's land.
They won't tell you why your account is frozen and the likelihood is OP will be told to withdraw, go away and not come back. They won't lose any money.
I saw a user here with problems with btc from casinos, a few months ago.
Thy closed accounts in the past
Take a look here
https://www.reddit.com/r/Bitcoin/comments/7bhf5v/coinbasegambling_site_what_exactly_happens_if/
Thy may have detected something not allowed with his account.