Since their family turned off the heat, it should be cold, the pipes must be freezing, and there is water in the pipes which means the water in the pipes is freezing causing it to burst.
In the extreme winters , the water inside pipes freeze, on freezing it's(water molecule) density decreases and volume increases and therefore, the ice then expands and pushes the water towards the faucet, causing high pressure buildup in the pipe and after a limit the pipe burst. This is what happened to the family in Ottawa, where winters are chilly.
