I may be confusing devices, but I remember watching a youtube video about iphones or another mobile device that over-heat. I also saw something flammable or maybe I am confusing it with a razer toy that people ride around on. But I do get the feeling that these devices are not being made as well as the used to be, which makes them very sketchy.
The lithium-ion batteries used in mobile phones, many radio controlled toys (like drones)and even those weird rideable scooter things they call hoverboards have been known to overheat when charging and cause fires. The latest major incident was the Samsung Galaxy 7 Note smartphone which apparently was rushed to get it out before the iPhone 7 launch and apparently 1 in every 25,000 had a bad battery so they recalled them and didn't do the proper QC on the replacements so the device is no longer available.

Personally I think its the quick charging that makes some devices more susceptible to this problem.

Blame cheap manufacturing and poor QC 😛
I dunno, the Note 5 had a 3000 mAh battery, the Note 7 was the approximately same size and had a 3500 mAh battery. They squished a whole lot of extra battery in there somehow. Maybe to get that much battery in such a small space they had to put the poles/electrodes too close together.