As a retired 11C (Mortarman), I would say that it was due to a cook-off of the charges,
due to an overheated Mortar tube. And improper dropping(Indexing) of round by Assistant Gunner.(Main reason)
Charges should never come in contact with Mortar Tube. Proper indexing( inserting round in tube) of round would have prevented this
And possibly contributing- Dirty tube
Not sure how many rounds were fired before this happened, but was a common worry.
The way to correct this would be as follows(depending upon Mission and War/Peace).
1) Let Mortar tube cool on own.
2) Pour water or other liquid to cool tube down.
3) Urinate (yes Urinate) on tube to cool down.
4) If available - change tube.( Finish Mission with different Gun Track).
5) Retrain Assistant Gunner on proper indexing of round.
6) Swab Tube often

Oz