This is more than making bottom jokes—it’s giving power to the powerless. You sit on your bum. You poop with it. You keep it covered up. It's the lowest of the body parts. And now Zack’s bum has claimed power and it's running around causing mischief. And that mischief is escalating.
So this snippet takes that power struggle and develops it, escalating the problems with new and interesting details each time. And notice how while each action is absurd, they all make sense if you buy into the set up.