Damp hay. no air flow.  Single rows or 3-4 wide?   Going through a sweat, & How much rain?     Years ago I had a trucker hauling year old hay to a buyer.  4" rain on the stack, told him to wait 3 weeks, he went back as soon as the ground was dry. Loads 2-3 ton heavier. Stacked it tight where he delivered. 4-6" rain later stacks lit on fire. wet bales in the middle had got hot without being able to breath. Then the rain sealed the deal.. The original stack in Wyoming dried out quickly 8% humidity & 10-20 mph breezes & looked good.