[segfaultbuserr]

The explosive doesn't rot, but the trigger mechanism does. When the trigger is good, at least its behavior is predictable and a technican knows the precise condition that the bomb will detonate, but in the worst case, of which the trigger is completed damaged, it's unpredictable - they don't spontaneously detonate, until the slightest disturbance.

[fhars]

„Good“ triggers were deliberately engineered to be unpredictable to make defusing harder and so you cannot assume the the area is safe just because the bombing has stopped.

[segfaultbuserr]

The most notorious example is a time-delayed chemical fuse, it uses a slow chemical reaction to detonate the bomb a few days later, as a form of psychological warfare. If the bomb didn't explode as designed, it means the chemical somehow got stuck, extremely dangerous to defuse, no matter how much prior knowledge one has.