It may seem ironic that something wet can actually cause a fire. However farmers have known for years, how critical it is to get the moisture right when baling hay. This is why hay is cut, then allowed to dry for a few days before being baled.

Wet hay, when it begins to decompose will put of heat, the chemical reaction continues and eventually a flammable gas is produced. If this happened out in the open, think of the smell fresh cut grass or hay, it’s not a big deal.  

However baled and compacted that heat and gas can build. Stack that hay, put it in a barn and you have some really good insulation, add in 90 degree temperatures and high humidity and you have the possibility for spontaneous combustion. Temperatures in the hay stack can rise high enough that a fire starts, even without a spark, spontaneous combustion.