Why do bricks fall apart around chimneys? Many reasons. A good comparison is to the nose on your face. When you go out to the beach for a day, it is your nose that usually gets the worst burn. Just like your nose that sticks out, the chimney sticks out above the roof line and gets the worst of the weather. Rain, wind excessive heat.
These factors are why bricks deteriorate above the roof line. Other factors that contribute to decay are heating and cooling of exhaust gases from your water heaters and furnaces. As the warm exhaust gas cools, it begins to condensate, and the moisture tears the brick apart from the inside out. If your chimney is showing signs of wear, call a mason or a chimney contractor. They can repair the bricks and mortar before it continues to crumble and begin a serious safety issue and expensive repair down the road.