Interrupt hack been working since 5.1 from what I can tell, perhaps from 5.0 and it still ain't fixed. If I was allowed to, I could even tell you the name of it, but I'm sure google knows the answer
Although another possibility is that the mage saw you running at him and just wanted to blanket you and it landed at the same moment you tried to cast a spell. Has happened to me multiple times as well and there's nothing wrong with that.