Over-discharge or mechanical damage are the two most likely causes. Both can cause damage of the separator (insulator between the electrodes), and if that happens, then the electrolyte can ignore and you've got yourself a pyrotechnic show. The same can also happen via manufacturing errors, although that's much less common (maybe one defect per several million cells produced).

This sort of "spontaneous" thermal event (which isn't so spontaneous, but is rather initiated by the type of events listed above) is why it's important to treat lithium batteries with great respect. Don't overcharge them, don't over-discharge them, and if they suffer any sort of mechanical damage, treat them like a ticking time bomb. And even if you don't think that any of this has occurred, store them in a fire-resistant container whenever they are not being used.

Never forget - there isn't much difference between a really good battery and a mediocre bomb.