And heat-shrink tube
contracts when heated.
Now what?
Everyone knows heat causes expansion. 
Anyway... wayner is kind of right; rock actually releases it's grip. That is, even rock flexes, and has relatively weak tension carrying abilities.
In my mind the key here is just like boiling water at room temperature (aka evaporation) one needs to consider the random distribution of molecular speeds. --the bulk may have an average temp, but each molecule is likely not at average energy (vibe speed). In the case of rock breaking ice, This internal energy dance allows localized creation of the more voluminous (pressure building) ice structure.
IOW, stop considering the bulk energy.
Here is a fun weirdocity, ice is slippery because molecules near the surface vibrate only up and down, making a molecular vibe-table. Crazy stuff, H
2O