Completely unrelated (since we haven't actually referenced the line in the last year or so) Benjamin has started saying "Wha happened?" exactly the same way while he plays with his toys. He'll have a car or an action figure in his hand, and it will crash or fall over, and he'll look at it and say "wha happened? wha happened?" as a part of his game.
I love the little things about our kids that just make you laugh out loud.