The solubility of a given solute in a given solvent typically
depends on temperature. For around 95% of solid solutes, the
solubility increases with temperature, in the temperature range
from about ambient to 100 °C.
http://en.wikipedia.org/wiki/Solubility#Temperature
Thus decreasing temperature of the solvent decreases its
solubility, thus crystals start to form?
I dunno how to explain in molecular perspective.