I don't know what happened here, I asked Santa for a new bicycle.
What do you suppose happened here?
"What happened to your regular UPS delivery guy? Well, he was terminated. Sign here."
What happened? You weren't a high honors student. THAT'S what happened.
Guess what happened to Shadowfax after Gandalf left Middle-Earth.
By "we're not quite sure what happened," they mean "we're not sure we want to know what happened."