[Spruce]

How does a grenade "partially" detonate?

[68skylark]

I wondered that too -- maybe the propellant went off, but not the warhead. (The warhead fuse would -- hopefully -- not be armed unless the round had been properly fired inside a rifled barrel.)

[PsyOp]

How does a grenade "partially" detonate?

Probably dropped it on the primer and and the propellent went off. Without a barrel to contain it, the casing would split or fragment, but the warhead would not go off.

The warhead is spin-armed, so that it has to travel a certain distance from the barrel, spinning as it does so, before it is armed. This is safety feature for just this type of incident.