That hill is really steep, it's really not much of a surprise they crashed with two people on a single bike. I bet experienced tandemists with a drag brake would think twice about riding down that hill. I don't know what to think about the bike that stopped suddenly. Doesn't seem like something that would happen because of a bike failure.
As far as the fires go, I imagine rad has almost enough business buying batteries that they could afford an agent to monitor the factories that make their packs if they don't already have one.