It's been suggested that a large impact at the exactly opposite point on Ceres surface, which has been seen, could have forced up this lone mountain. It would supposedly have done this by sending shock waves directly through the planet.

A more far out suggestion would be that the mountain is a heap of tailings, left over from an extraterrestrial mining project.

For the first one to be true I think Ceres would have to be big ball of salt with a rocky surface. Otherwise, this focused shock wave just happened to pinpoint a huge isolated pocket and push it out. Not too likely.

Looks like that first idea has more merit - it's sounding like Ceres really is a big ball of salt with a rocky surface.
So much for my comet idea.