It has to do with heat. Movement in real life creates friction which developed heat. In the world of electricity, a similar thing happens because every material will absorb some electrons. When you absorb energy you create heat. So as the materials in the battery have more and more current pushed through then the hotter they get. If too much current is pulled through the battery then the matria will get too hot.
In the case of modern lithium ion batteries, that heat will begin to melt the lithium metal. Hot lithium metal bonds with oxygen in the air in a rapid and explosive release of energy.
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