Why do water pipes burst in winter?
Water expands as it freezes into ice. In a pipe, ice forms first on the inside wall of the pipe and grows radially inward until there is a solid plug of ice blocking the pipe. Until that situation occurs, the expansion of the freezing water in the pipe merely pushes water back into the water main. When the plug of ice completely blocks the pipe, it seals water between the plug and the closed valve. If more ice forms between the plug and the closed valve, the expanding ice has nowhere to go, and causes the pipe to burst at its weakest point.
Do hot water pipes burst more often than cold water pipes?
Hot water pipes are more likely to burst because the initially higher temperature of the water in them makes the formation of ice crystals more difficult. The water in the pipe then supercools, that is, goes below 0°C. Then, when freezing does occur, it occurs rapidly, quickly blocking the pipe and trapping water between itself and the closed valve. The sudden formation of the plug causes the pipe to burst.
This information is taken from The Flying Circus of Physics with Answers by Jearl Walker, published by John Wiley and Sons, 1977.