It does not take much moisture to start the fermentation process. Once the process starts the heat differential creates more moisture and the cycle builds. This is why a bail of damp hay put into barn can cause a barn fire over time from the heat produced by rotting process. That's why I separate my feed and don't keep it in bulk.
@aart is right also in that the age of the feed cam contribute to the problem. The feed either came moist or somehow moisture is getting into it. I would look to control the entry of moisture if it is happening with more than 1 bag of feed.