The Core Material suffers from thermal runaway. Once it reaches the critical temperature there is no going back. They burn so hot the adjacent cells can go off in a chain reaction. The trigger can be overcharging, external heat, physical damage, or charging failed or under voltage cells.
They look like little highway flares, or those fireworks that sit on the ground and shot sparks and fire.

Lithium Polymer will still burn, especially if you puncture it with a metal object.
Some Lithium cells will not burn, but they have a lower energy density.