Maybe it is because of an earthquake? :)) Just joking.

On a more serious note, I'd say it happens sometimes...Due to extreme heating/cooling or due to poor materials or manufacturing defects. I had a similar problem, called a professional and he said it happens sometimes. No specific reason except the ones above. I was amazed as well....

Here is a link that might be useful: Brick Fireplaces

You're burning haunted trees. :-)

I've heard of this happening, too.