Power, temperature or simply a crash
Q-6
Need to get very hot before it will shut down, CPU will as a rule throttle down first. I would just restart and try to replicate what you were doing. Like as not is will just be one off instance, if it repeats call Apple.
You can also run Apple hardware test, I would just tend to use the computer and see what happens, I have had Mac`s shutdown, however very infrequently.
https://support.apple.com/en-us/HT201257
Q-6