She had recently had fresh hay delivered; and apparently it was still curing, was packed very tightly in her loft and we're in a record heat wave - conditions that produced combustion.
Fortunately all the horses were out; but the barn (and all the tack!!!!) was a total loss.
PLEASE make sure your hay storage has adequate ventilation, and please make sure you leave adequate space for air circulation around stacked hay bales. Try to avoid storing newly baled, still curing hay.