irrelevant, unexpected, unusual. or for a more technical definition, selected on no particular attributes.
i can't think of any. here, i will insert the word random.
Something irrelevant. Randomness or surrealness can be humurous. Some beleive using "random" words such as CHEESE or CHOCOLATE MILK is funny.
not random: flying sandwiches!!!!!
random: a boy in orange suspenders marches up to you at lunch, grabs a hostess cupcake from his tray and mashes it in his eye.