I bet they just don't want you withdraw all your BTC
And they want you to play more at their casinoo, so you will lose the games and your bitcoin
Even some dice games (999dice.com) have problem if you withdraw too big
Most likely, they are scamming you & other player
Some sites I know of, used to put withdrawls on pending, but would process it manually, and till it wasn't, they would leave it in your account till then
and a lot of people would just gamble is again and lose it.