Since plant matter generates heat as it decomposes, where would a fire be most likely to start in the piles of grass?

In the middle of the pile of grass.

Explanation:

The fire would be most likely to start in the middle of the piles of grass if spontaneous combustion occurs because the heat doesn't easily move through the medium due to the insulating nature of grass. A pile of grass can catch fire due to spontaneous and quick combustion in the middle of piles of grass. So the fire starts in the middle of the pile of grass.