My guess is that the crust is think and strong, and that they heated up the pizza too much to where the weaker and thinner parts melt or something and then the now-molten cheese and stuff goops down and forms a delicious stalagmite. The center is the farthest away from any support, so that'd be the place that caved in first, unless there was a serious deformity elsewhere like an intentionally drilled hole.

Pizza science!