It is because of the pressure inside the pipe.The water molecules present in the pipes freezes.As the ice expands,it pushes water towards the closed faucet. This cause an immense amount of water pressure to build between the ice blockage, and the faucet- eventually,the pipe ruptures under the pressure, usually at  a spot where there is little or no ice.......
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       Water pipes may not burst during summer.  They may burst during winter.  The reason is that as water cools to 4 degC, it compresses to highest density.  Below that temperature, water expands.  Ice has a higher volume than liquid water.

     The pipes made of metal or fibre shrink a little due to the cold.  So it is possible that elastic limits of the pipes are reached due to the pressure exerted by water on the inner surface of pipe.
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