First of all, a Blackberry's processing power is nowhere near a 1GHz Snapdragon, so the power to idle is much less than your Incredible. Throw out any other comparisons you're thinking of using between the 2 phones.
If you left your phone in your confined glove compartment, where the inside of the car is probably near 115 degrees, your Incredible has nowhere to vent the heat from its already hot processor, which is now building more and more heat from external conditions. You let this happen for 4 hours; would you leave your laptop running at those temperatues? I hope not, that's how thermal paste dissipates (Xbox 360s Red Ring of Death) and processors and GPUs lose their connections with the motherboard. I'd say this is probably what happened with your CPU both times.
I also think that its complete common sense that the place you had the phone stowed was the culprit of why 2 perfectly working phones just stopped working, being as how both times you went back to the car to find the phone broken.