I'm guessing it's because everyone makes fun of his feet. His feelings must be hurt.
If someone called ME bigfoot all of the time I guess I would go into hiding too.
He must be really self concious.
C'mon guys, let's make him feel welcome.
Let's call him something like: Big Dick McGee.
That ought to make him feel better.
(And Monkey, "Bow to your Overlord")