It seems you found the reason already. Anyway, I also suspect something exceeded the SMC limit. May be something overheating, may be a connector (e.g. mini 6pin) overloaded. Therefore, the SMC commanded a hard shutdown protection.
Once this hard shutdown protection is activated. A SMC reset is required before you can power up the cMP again. That's why you need to unplug the power cord first. (It seems Apple force the user will disconnect the power cord, which facilitate them to open the case and check all the hardware before power up again)
The reason why it only happen in Windows, most like because you only really stress the card the max in Windows gaming.
Thanks again, Martin. Was a cheap connector and was a long time since checking. One black wire separated from the connector. Had the same shut down occur on a cutscene with another game before so I just skipped it and avoided it and played the game without it. This one was xbox app loading Mechwarriors 5, that one as soon as the company splash shows it happens there. Some games must be programmed to just draw more power from the card at spots.
Good to have it more healthy lately, yes was kind of forced to take better care of. Getting the NB cooler, and not needing to run the fan at higher RPMs and still having it cooler than before. Just redoing the thermal paste and substituting some bolts instead of the cheap plastic push pins. I never did it when upgrading to 6 core many years back.
Even unhealthy did pretty well considering that. The only real problem ended up being was chrome and a bad extension that caused the system to lock up.
Rather impressive prolonging of support compared to the 1st Mac Pro, was only about 4 yrs Lion to el capitan. Whereas this one support ended at el capitan and goes to Monterey 6 years.
I dont know about the one in the middle never owning one...