In case of thermal runaway of a Lithium-Ionen-Battery short-circuits often occur inside of the battery cell.
This short-circuit leads to electric arc discharges and heat. By this phenomenom the electrolyte inside of the battery turns into vaporization. These vapour/gases are highly flammable and lead to the presence of a ignition source to a fire or explosion. Those reactions depents highly on the charge level (so called Status of Charge “SOC”) of the battery. The higher the SOC the stronger and faster the reaction proceed.