...Lots of crazy stuff happens below. The expression 'when pigs fly' means 'that is COMPLETELY CRAZY, and it will happen when pigs fly.'
(For those of you with extremely limited zoological knowledge and who only encounter pigs when they're on a plate in bacon form, here's a hint: pigs don't normally have wings. Or jetpacks. Or propellers.)
So if pigs ever did fly, all that crazy stuff people had put off until they flew would happen at once, from a laser-wielding knight to a very literal interpretation of a dogwood tree in a dinghy sailing down the street.