One of the questions I get asked from time to time is, “If the ball is called a ball and not a circle, why isn’t the square referred to as a cube and the triangle a pyramid?”
The idea is that the Ball is the only character with depth to his personality. The bullies are just one dimensional, hence the flat geometric terms. The only hint of the bullies having any “depth” to their soul is in the shot after the credits when the square tries to literally move forward in it’s life. In many way’s that could be the start to the next film.
Plus, I just liked the idea of calling the bully a “square” because of the slang aspect. That way when the Ball tells them off at the end and say’s, “I’d like to see you bounce, you square” it has a secondary meaning.