Why do liquids freeze into a polycrystal solid when they freeze fast? I don't see any possible explanation. If there's a crystallite at one orientation, why would a crystallite at another orientation start growing instead of more molecules just building onto the surface that's already there in the same orientation? Does the size of the crystallites vary as the reciprocal of the speed of movement of the boundary between the solid state and the liquid state?
Embryo crystals spontaneously and independently form in a supercooled liquid. A crystal successfully growing in one location doesn't prevent others from growing elsewhere. As the temperature decreases, the rate of embryo formation and chance of successful growth increase, so at a low temperature you're likely to get multiple successful crystals.