Some other questions:

1. Did any external moisture get into the stacks? By "external moisture" I'm not excluding anything. Hay can wick up quite a bit of moisture from the ground in a barn if the hay isn't on a floor or a gravel pack. Even in a barn, hay can wick up quite a bit of moisture. I could show you hay that is stored here in barns in Nevada, where we get bupkiss for moisture, where the bottom 2" of the bales are damp and moldy. I can only imagine how much moisture comes up through the dirt floor of a barn in Tennessee.

2. Any leaks in the roof?

3. Did anyone take a moisture probe of the hay when it was baled?

4 to 5 weeks between baling and ignition is about the right time span for hay that was baled with more than, oh, 22 to 23% moisture.

How hot has it been there (both day and night temps)?