An internal short forms when something compromises the plastic or ceramic-coated separator. This can be something beyond your control, like a latent defect, a lithium dendrite, or damage to the cell (i.e. metallic burr, nail, bullet, crush, etc.).

Solid foil current collectors dump the battery's energy into the short, generating heat.

Heat causes the plastic separator to melt and shrink, opening a bigger short.

The battery just stops working right? Wrong. The solid foils continue to dump energy into the short, creating a positive feedback loop that results in FIRE, EXPLOSION, or a BIG BANG.