The simple answer is because they have not been properly insulated to protect them against the cold. But don’t go telling that to your mum and dad when they are trying to catch leaking water with buckets and towels after a burst in the roof!
The scientific answer is that ice takes up more space than water. If you have ever seen milk bottles standing outside on a freezing morning, you will know that the frozen milk pushes the cap off the bottle. The same thing happens in a water pipe, only there is no cap that can be pushed off to relieve the pressure. If ice forms in a pipe, it presses against the inside. If the pressure is great enough, it bursts the pipe. What is annoying is that you do not know that this has happened until the ice melts and the water starts flowing again. Then you find out soon enough as water leaks out of the split pipe, drips through the ceiling and makes a mess in the rooms below.
Picture Credit : Google
