Between the other thread, and the comments on both videos (he has a follow up), it basically boils down to the fact they were using a dumb charger and the packs were out of balance. No BMS was used either. So the charger overcharged the cells until one of the packs went thermal. Because of the way they were packed into the cart, the heat was contained and caused the others to overheat and go thermal.
This all boils down to stupidity and blatant ignorance and over confidence. Just like Sean says, within their guidelines and not abusing them. That simple.