Why are bales more likely to go mouldy?

The grass in bales is not so highly compacted as in a clamp and is not chopped as short. 

These factors, together with the fact that the grass for bales normally has a high dry matter content, mean that more air is trapped in the bale initially, allowing the yeasts that initiate aerobic spoilage a longer period to grow and multiply before the air gets used up.  This means that there are higher numbers of yeasts present later if air should get in so spoilage will begin faster. Bales also have a high surface area to volume ratio which means that a greater area is potentially exposed to air. About one third of the volume of a bale is contained in the outer 3 inches.