You have a large mass of grain warmer than outside temp, heat rises and when it meets the cold air you get condensation.    The condensation creates a layer of spoilage.


I know more than one guy that got complacent last year with dry corn going into bins and didn't run bin fans enough to cool it down, including myself.    Some of them had a lot of trouble with unloader sumps getting plugged by chunks of spoiled grain, thankfully my crust wasn't thick enough to give problems.


You can get away with putting grain in a small bin (think govt bin or similar size) without a fan but it needs to be later in the fall and cooler weather to get away with it.   And it needs to be leveled if its not going to be moved/used soon.