Holy shit dude. If a crashed jet, spilled out burning fuel that created an extreme environment that changed the normal storage conditions of a munition, to which they then detonated - then no, they probably weren't improperly stored. It was the plane crash that cause the whole event.

Your assumption is that over time there should have been a mishap event that should have led to an accidental nuclear detonation. I'm saying that nukes are not stored in a fully armed manner, and have enough failsafes to where if some catastrophic event happened, the nuke would simply be non-operational to where a fission/fusion reaction wouldn't be possible.