It was a known issue with the first gen 1070/1080 cards. Something about inadequate VRAM cooling. EVGA put out a notice that this was an issue and they would replace the bad part for free but I apparently missed the memo. They also pushed out a BIOS update that would prevent it from overheating but I missed that one as well.

Presumably EVGA didn't do forensics, and if they did, they may not have had incentive to publicize the defect.

Suffice it to say that most materials are flammable if they get hot enough. And there are plenty of reasons why a circuit board with many components hooked up to a power supply might get really, really hot. I'm surprised electronic components don't catch fire more frequently.

Slowly and then very quickly.

Presumably sometime after the magic smoke escaped.