I'll play fair on this one:
I'll assume no user error or user intent to blow the packs.
This was a stick pack! So the cells are most likely not matched. One cell could have been a dud and during the charging process the weaker cell could have masked the real delta peak. The goober cell could still be taking a charge even though most of the current going into the cell ends up as heat not stored capacity. It is just a matter of time before 5 good cells become ticking time bombs with 15 minutes of overcharge. I would think it would take 10minutes before the bad cell will "peak" and the run-over charge after delta peak in some chargers is 5 minutes (normally 3min). With that 15min at a 4amp charge the batteries are getting 1000mah more capacity than needed and increasing in tempatures at the same time. At 1000mah too much capacity that would be more than enough energy to boil a liter of water from room tempature! Depending on the stick casing and which cells were cooking off the temp prob could completely miss the warning signs. This all leads to boom!
In reality I think the goober in not the stick pack, its the user. There are just too many things the user can do to detonate batteries by accident or by intent. If the battery was going to fail, normally it has already done so before the user even places the order for it.