Mulch/ wood chip pile fires are very common. I respond to numerous ones a year.
They can get hot enough from decomposition that they will self combust. Biggest problem is, the fire normally starts deep within the pile. As someone stated above if you don't break it apart you are effed. If you break it apart you are effed. It just gets bigger until enough water is placed.
This is one we had back in May of this year.
