Just goes to show the importance of the Battery Management System..
Thermal runaway refers to the condition where the battery will not cool down anymore by disconnecting it and throwing it in the freezer. In lay men's terms, it has caught fire, which will inevitably result in venting with flame (explosion).
How do you reach thermal runaway? Heat any part of it to 130C (some lower, some higher). So just slap temperature sensors on it and cut it off at 70C? Not so fast. It's enough if a small part of it reaches critical temperature.
Many fires are the result of impurities introduced during manufacture. A small dust speck of metal inside will heat up as current passes through it, and make a small part of the battery much hotter than the rest.
The low end and high end if the li-ion voltage range does funny things inside the battery cell. The lithium-ions can develop into metallic lithium, which is not reversible, and cause much the same issues as with impurities during manufacture, with the additional bonus of possibly puncturing the separator, causing an internal short circuit.
The copper used to collect current can dissolve into the electrolyte, and later when it becomes solid again during use, it will appear in random spots in the battery, again acting as a spot heat source and agaib possibly puncturing the separator.
All this is accumulative damage, a BMS squeezing too much capacity out of a battery cell can gradually make the battery too unstable for use, which results in boom. Most likely during charge, because that's when we see the highest sustained currents normally. There's nothing preventing it from happening during use though, so just because a suspect battery charges without incident, don't relax.
A properly tuned BMS will be causing just enough damage over time, so that the usable capacity of the battery will have degraded enough that the user stops using it, before the battery's stability has degraded enough to explode under previously safe conditions.
In other words, li-ion is a ticking time bomb, but manufacturers try tune it to explode way after your device is obsolete.