Possibly the phenomenon known as “thermal runaway”is most likely cause. However a defect in the manufacturing process, stress on the battery, such as excessive vibration or even an electrical fault are the main ones. Or it could just be a fluke, a luck of the draw where a vehicle spontaneously combusts.
The fire onboard a ship in 2019 was caused by thermal runaway when an I pace went up in flames!
You may find this interesting on TR.
https://onlinelibrary.wiley.com/doi/full/10.1002/aesr.202000059