My understanding of these battery fires is they are caused when using an after-market or different charger for the battery than the one it came with.
There are standards that chargers are supposed to meet now, where they negotiate the voltage of the battery and determine state of charge.
But spontaneous combustion of smaller batteries isn't unheard of, even when they aren't connected to a charger. I remember a few years back a story of a phone igniting in someone's pocket.
But again, these stories are far more rare than those we hear about with E-bikes and cars. Remember when GM advised poeple to park their Chevy Volts (or maybe it was the Bolts) outdoors to avoid fires in the garage?