With all due respect troy (ye of veteren status to my measly newbie state), I believe that the contraction and constriction of the piping material has more to do with the breaking of water lines.

In addition, you may very well have ice in the lines prior to the break but it is the warming of the pipes (and consequently whatever is in them) that causes the break. That warming will likely be enough to have facilitated the change from ice to water at this point. The outside air (even underground) will likely be warmer too than the ice plug and thus the temperature change will quickly melt the ice and allow the pipes to 'leak'.