To quote one of my posts about Sanyos from 2013-2016:
My current hypothesis is that the the lithium concentration inside of the LiCo2 anode is higher than usual, and leads to a phenomenon called passivation layering, along with the lower purity of the cobalt compound, caused the problems we encountered in these cells. It's still not certain, but I could come to a conclusion in a few weeks.
Basically, this usually happens in lithium primaries due to their very high molar concentration of lithium, forming a passvation layer as the battery isn't used. This isn't really bad for them, as they aren't rechargeable, meaning it isn't a problem. In fact, it lowers their self discharge rate significantly.
I came to that statement, because I noticed that with my Sanyo cells from that time, their self discharge rate is very low, even for a lithium ion battery. Even my best new cells didn't have that low of a self discharge rate. So, I did some more research, and apparently, that behaviour is quite bad for lithium ion batteries on the first cycles, as by clearing that passivation layer, the internal resistance is quite high in the beginning, resulting in much more heat than usual if a battery was stored for a while. With the lower purity of the cobalt used, it resulted in an actual lower thermal runaway threshold.
That, along with charging a possibly months old pack along using the computer in a high heat environment, lead to the thermal runaways, and the massive recall.