Anything, but usually the casino didn't give a clear excuses, they will say the account is under investigation either by casino or third party gambling provider.
I know it sounds strange, but not always ToS has all the necessary information. Some years ago I have won in competition and received several freebets (some twitter competition). Some of them I have lost, some one, but when I have decided to withdraw, my withdrawal was pending for days. The reason for pending was wagering requirement, which was not stated in ToS and competition rules. I have found it only from a chat with support. The story ended with me loosing funds, because I have failed at wagering x5 my freebets (I had to wager x5 all 3 freebets of 1mbtc).
These rules and requirement of new competitions can't be found in TOS including in bonus terms. Either it's explained on the Twitter or the casino will state it on the casino. It's really strange that the casino didn't state it while you claim the free bets.