Why *is* there a huge mountain on Ceres?

I’m sure if you’re reading this, you’re already regularly checking out Bad Astronomy, but just in case you missed it, Phil Plait is asking a pretty fruitful question about 3-mile-tall Ahuna Mons – Why is there a huge mountain on Ceres?:

There’s nothing else quite like it on Ceres, and I wouldn’t expect there to be (to be fair, there are other hills that are similar, but nowhere near as big). Ceres is about 950 kilometers across, with a thin rocky crust overlying a thick water ice mantle and a rocky core. The interior isn’t active like Earth’s, so mountain building isn’t something you’d expect. There are features on the surface indicating the crust has experienced some tectonic activity (cracks, depressions, ridges, and so on), but that was long ago.

Maybe there was a crack in the crust there, or it’s just thinner. Water ice from the mantle squeezed up, forming the mountain. Maybe the impact that formed the big crater nearby helped crack the crust down to where the ice could leak out.

Again, I am totally speculating here. Whatever Ahuna Mons is, it’s weird.