Everything falls faster than a comically heavy object
Whenever characters - particularly cartoon characters - fall through the air alongside some sort of extremely heavy object, they will inexplicably fall faster than it does, regardless of their own mass relative to that of the object in question. This is done so they can humorously crash into the the ground, leaving a character-shaped outline, then crawl out just in time for the object to fall on them. The classic example object here is, of course, the humble anvil, but this also applies to boulders, falling structure work or the character's own mechanical device that got them in this position in the first place. Being in this position pretty much automatically qualifies the character for the term Butt Monkey, probably Iron Butt Monkey given the inevitable consequences. Why does this happen? Because Rule of Funny, and because Gravity Is a Harsh Mistress. Up for Grabs.