I think this is because the particles don't know or care about each other, and they act completely without any peer pressure. The direction in which any one particle vibrates is completely random, and there is no connection or influence among the particles. That means that any direction is just as likely as any other direction for the next vibration, and they all wind up vibrating in different directions. There is a tiny tiny tiny tiny chance that all of them could vibrate in the same direction for just an instant; if that ever happened, the rock would suddenly jump up in the air. That's actually true, but the chance is so tiny that it hasn't ever happened yet. In fact, the chance is so tiny, that when scientists do their calculations of particle vibrations, they assume that the chance is zero, and that makes the calculations simpler.
