I'm not really sure. I do know that there are a few ways that a detonation of a mortar round can happen like this. The video does not say how long they have been lobbing motors. If the mortar tube was fired for an extended period of time it could overheat causing the round to detonate. I don't think this is the case.

Another problem can be with the delayed fuse. If this is not set or set improperly then detonation can occur.


But considering this is the type of weapons stash that we would find....
You can see a potential problem




There is something else that could have happened. We have the technology to determine were a round (artillery shell) is coming from. With pin point accuracy.
If they or other teams have been in the area some time lobbing rounds with little movement, we can put a round right ontop of that artillery group. Not sure if this is the case either as I did not hear a incoming round.