I am making the assumption these were either supersaks or 25kg bags. (AmNO is readily available both ways.)
The both forms have an internal plastic liner, that should do a pretty good job as a moisture barrier. Of course, I don't recall ever seeing powder or crystalline goods sitting in inventory for 6 years! So, that's a substantial variable.
It's seaside, but warm, and the hygroscopic state of AmNO requires a relative humidity of 59.6 or 59.8% to hydrate. That's pretty high humidity given sea breezes. Of course, this was inside, so maybe it could get that high.
BTW: The solid isn't more dangerous from an explosive stability standpoint. However, since the material is shock sensitive, it becomes more dangerous to work with! Breaking up big chunks carries some risk because enough mechanical force could break enough of the lattice and release energy sufficient to initiate decomposition. The decomposition is what we otherwise call an explosion!
Generally, in industry, they do not recover solidified AmNO. They place the blocks in water, redissolve into a solution near the saturation point and redry to the powder. To dangerous to try to grind it back to a powder.
We didn't even store 500g jars of the stuff on any shelf near ANYTHING that could burn or undergo exothermic reactions. Basically, we stored it next to silica or alumina gel, or simple salts like sodium chloride or sulfate.
It's dangerously stupid to do what they did.