For the layperson, this means the battery is shorting internally, commonly called a internal shunt. When this happens, the battery dumps its fill load of current across that short and temperatures internally skyrocket, much the same as a space heater does when plugged into an AC outlet. Shorting across a power source creates a complete circuit and essentially becomes a miniature arc welder or heating element. In home electric wiring, a short can burn your home down (as did mine on December 12, 2015).
This can happen from several causes. It can be due to excess compressing, folding, bending or puncturing the battery. It can also happen during manufacture, when as one of the last steps before the battery casing is heat welded shut, the electrolyte - the Lithium paste is injected between the folds of the battery sheets, but doesn't coat them completely. Then when the battery is sealed and compressed flat, the two internal "poles" of the battery, the Plus and Minus poles, if you will, come in contact or are close enough that an internal shunt can be created easily through normal charging.
These internal shunts can also happen when the battery is charged improperly. This is one reason why the charging process of the batteries are so critical and they are so carefully monitored and administered. In the case of these phones, the issue is related to charging, so I suspect it's maybe not so much a battery issue as an actual charging issue. This is also one of the reasons I tout so strongly that you use only the charging adapters and cables supplied by your phone manufacturer or the ones sold by the carrier who markets and sells your phone.
I'm wondering if maybe there is a flaw in the charging. With these phones being forced to charge faster and faster, the risk of batteries developing internal shunts grows higher. Then you have a few batteries that may have marginal electrolyte coating and what you have is there perfect storm.
In this case, they claim these batteries (some of them), have apparently suffered some serious flaw in manufacture and this contributes to some (a very small percentage), failing while charging. It is a very dangerous risk and nobody should be continuing to charge these phones against the manufacturer's advice. Instead, when you are able, take them, in a metal container to the nearest dealer for exchange/replacement. Using them may be OK, but under no circumstances would I charge them.