If I understand correctly, atomic weapons have a "strong link" and a "weak link". The detonation won't happen without everything firing correctly and on time; the signal to do that has to pass through both the strong link and the weak link. The strong link isn't in place until the bomb is armed. But couldn't a violent-enough accident knock the strong link into place? Yes, but that same amount of violence would destroy the weak link. (That's the point of the weak link.) So even if an atomic bomb had an accident hard enough (and lucky enough) to arm it, it still couldn't explode, because the weak link would be broken.