A thermal runaway in a lithium ion battery can be induced by either a heat source or a short across the terminals. Once one battery overheats it will induce a thermal runaway in other batteries close by.
This is the reason that there are strict rules about the carriage of these batteries. Some types of battery are CAO (Cargo Aircraft Only).
Having undertaken practical training in dealing with lithium ion battery fires, I doubt that this is the cause of this tragedy. These batteries are extremely explosive when in a state of thermal runaway so I doubt that the aircraft would continue to fly for several hours after the event.