If you have a big enough pile of organic matter, exothermic decomposition can lead to fire, if the conditions are right.

Commonly happens in grain silos, mulch piles, etc.

It really doesn't take that large of a pile of grass clippings or wood chips to get some steam coming out and a lot of steam if you disturb the pile.

Yep, many hay barns have caught fire because the hay was bailed while too wet and it is fermenting so vigorously.