AN needs to be mixed with an oxidizer.
Oxidizer = DEW
Ammonium Nitrate needs to be mixed with an oxidizer in order to detonate.
That's why they felt comfortable leaving it there for so long.
The clear message to take away is this:
There is no longer such thing as safe long term storage of stable/inert chemicals.
Directed energy weapons (DEWs), which have been used up until this point to modify the weather and cause massive wildfires, have now been used to oxidize stable/inert explosives.