A damaged, poorly manufactured or overheated cell can keep heating itself up: thermal runaway. The cell then releases a lot of heat in a short time, jets of flame and toxic, flammable gases, sometimes with a pressure wave or flying parts. The heat spreads to neighbouring cells and to other batteries nearby, causing a chain reaction. Such a fire is hard to extinguish and can reignite after it has been put out.
The risk increases during charging and with batteries that have been dropped or damaged. If you store or charge several batteries at work, a cabinet tested against a fire inside limits the consequences. How such a cabinet is tested is explained on our page about fire-resistant storage of lithium-ion batteries.