Usually charging is the culprit for an intact battery, but if it has been damaged (and this is rarely obvious from the outside of the pack) it usually just needs a little time before it splodes. Inside the pack the cells can become an accordion due to impacts/hard landings or the tabs will split or crack. From what i have seen its best to just put them in an area that you are comfortable having a fire and hopefully be pleasantly surprised rather than running the loosing property or even (heaven forbid) life. I charge mine in the oven in all honesty lol. I figure that is a pretty safe place. Oh one thing is that if you ever spot a performance change from what you are used to (increased/decreased power/run times) then i would be on guard, i don't know the science behind it well enough to spout facts but that seems to be one of the only warning signs that they give aside from puffing that something is wrong. I noticed you said that the batteries did well just before the incident were they usually didn't before. It seems counter intuitive for extra power or run time to be a problem but it seems to mean that architecture of the battery's interior has changed is some way which is never good. As TLC said it can sometime be down to the factory and luck as well!