I would imagine that the proliferation of cheap devices using lithium batteries also contributes to the issue somewhat. While anything with a lithium-ion battery is susceptible (as seen with the exploding Samsung Note debacle), bargain bin electronics are almost certainly not selecting batteries to as high of a standard as something with better profit margins, like an ultrabook or flagship phone.

Cheap chargers also likely contribute. There's limits to how much charger misbehavior even the best batteries can tolerate and some of the charger bricks sold on Amazon, in gas stations, etc are horrifyingly bad.

There really need to be rules in place to prevent these ticking time bombs that are cheap devices with 0 safety considerations or QA from being brought onto planes.

Yes, cheap devices are real problem. China has strict rules for transporting batteries that are actually enforced.

Also no lighters on plane, because they know in practice they will be used in lavatories. They will find a lighter, or even just a spring from a lighter that fell apart a long time ago. Beijing airport security is quite impressive actually. They probably won't notice the knife you accidentally left your bag, however.

One contributor is that most batteries have a protective circuit built in. It usually has a DW01 chip and a pair of MOSFETs to prevent overcurrent charging, overvoltage, undervoltage, and overcurrent discharging, all for under a cent.

Since most batteries are protected, the cheapest chargers can be 'dumb', with a simple constant current output. When the battery is charged, the built in protection device will stop the charge when charged.

However, only most batteries have this protection. If you plug one of these cheap dumb chargers into a battery that doesn't have the protection circuit, then it will catch fire after a few hours.

The usual culprit is 'replacement' (non-OEM) batteries for an expensive device, for example a drill. The OEM version has a smart charger and a protected battery. A non-OEM battery lacks protection. A non-OEM charger is dumb. Either alone is safe. Combine them, and a fire happens. And because the device is expensive, the people in China who designed the battery and charger never had the original device to do proper testing with.