Why did Augest get stuck in the pipe ? what finally shot him like a Bullet through the pipe ?

Mr. Gloop gets ready to go in after him, but Augustus is sucked up one of the giant chocolate pipes. The pipe, however, is not big enough for a boy of his size, and he blocks it up. The chocolate begins to build up below him until the pressure becomes too much and finally, like a rocket, he goes shooting upward.