Chimneys take a lot of abuse from the elements, especially here in Canada with the cold, snow, wind, and rain.
All it takes is a small crack to start allowing water to seep in. This moisture goes through the freeze/thaw cycle, and results in flaking and rotten bricks, otherwise known as “spalling bricks”.
It’s important to do regular maintenance on your chimneys in order to ensure the top of the chimney is watertight, and properly domed to promote water run off.