~snip~
I do use a VPN but I didn't use any tips or have insight in a secret seed. Do they do this to everyone who wins a few grand? Although my cashout was large my profit is only ~4 bitcoins.
Yeah sometimes they do this, and your case I have seen before. It doesn't mean you've done anything wrong but they just take some precaution. If you profit only is 4 btc and you deposited 31 btc than they might think you try to launder your btc through their site. But all this is speculation.