Some guy farts and says, "That was some asshole behind me."
Blaming the Fart
Explanation
The humor comes from the guy blaming someone else for his fart, implying that the 'asshole' is a person behind him, when in reality, the fart came from his own 'asshole'. It's a funny play on words.
Teresa
I've heard it this way: When you hear someone fart you say "hey, there's some asshole talking shit behind your back" lol