Are you sure you don't have any alternate account or something that got limited in the past? Obviously you won on a very big event, so the problem can't come from that.
Even if that's true and the casino believes that the user actually have an alternative account or probably something else the question still remains: what exactly did he do that made them to come to the conclusion that he violated their terms. If they think that he had another account or maybe he had violated a particular rule, then they should at least mention the rule and tell him what they discovered.
Just simply telling a user that the sportsbook activity broke their terms without any valid explanation followed with evidence make the situation look fishy to me. Okay now let me ask, if he had lost the bets would the casino still cancelled the bet and return his deposit just because they believe he violated their rules?
But it's actually a good thing that they returned the original deposit to him, but I don't think that's a good replacement for a proper explanation of why the winning was cancelled.