In my opinion, 3x4x8 is a pretty big bale for that kind of moisture . (14-15 percent) The moisture can't migrate out.  With innoculant (buffered prop) that may have kept.  Did your stacks sag or tip at all?  That's usually the first indication you have a problem. It also helps a little bit to not stack them tight against each other to let the bales breathe and moisture to migrate out.  Also agree on comments about covering and store inside for big squares.  I would be curious what a moisture and temp probe would have showed before it rained.