It's derived from a Mitch Hedberg comedy act:
"I mumble a lot on stage, I'm a mumblerer. But sometimes what I mumble is some insignificant shit. Like I'll be walking down the street with my friend, and I will have said something, but he didn't hear me, so he says "What!?" SO I'll say it again, and again he had not heard me, so he'd say, "What!?" And now by this time I am yelling "That tree is very far away!"
Person A: (mumbles) Smells like popcorn in here
Person B: What?
Person A: Nothing
Person B: ???
Person A: That tree is far away