Stunna just said in chat that the guy abused an exploit and therefore could use an unhashed seed; so he could predict the rolls. Obviously won't be allowed to withdraw.
Why people abuse exploits like this?
He could earn some buck either by telling Stunna about the bug or by moderating his luck, but the way he did he got no winnings
He withdrew 40 btc and probably took a lot more without detection before this run. The thing is, there is no amount Stunna could really offer to make this worthwhile financially for any exploiter, since he can take a great deal more through theft.