In short, it simply depends on an asteroid. This one had ice, another might have not.

Yes. Imagine a dirty snowball. As ice on the outside burns away the thing shrinks until the outer layer is more rock and less snow. So you have a layer of dirt/rock layer above a "core" that has more ice. But he ice inside is still being heated. So ports, like little volcanoes, form in the outer layer to let steam out from the core. Thats where the jets of water/ice come from. But it all starts with the dirty snowball.