RE:Win At Life cant quote yet I believe. Was a lurker for like 10 years before I registered up.
Essentially there are usually two types of failures on these devices. Usually the bad ones happen on the more advanced ones that have rechargeable batteries being used.
Example 1: the atomizer (part that is being heated up) is not the one intended to be used by that rig/setup/whatever. Too much power is poured into the thing and it will POP, but the pop is usually minor unless someone is really really pushing their luck, happened to me before using wrong resistance atomizer.
Example 2: someone is using rechargeable batteries that do not have normal safety features to shut them down when an internal failure happens due to extreme heat/pressure caused by usage outside of normal parameters. A runaway reaction occurs that causes blistering up to explosion.. I've seen one of these go, dude had the outer casing half exposed and barely got it out of the mod he was using before it started the blister. Some of the less reputable although cheaper available batteries do not have safety features
https://www.livescience.com/50643-watch-lithium-battery-explode.html