Previous shutdown cause 0 on Mac Pro 2008
Considering it is an eleven year old machine, I can quite safely say (from three decades as a hardware tech) that either the power supply is starting to play up, or one of the voltage-regulator blocks that keep its Xeon(s) well fed are starting to get constipated with age. Either scenariou is equally plausble.

When was the last time the ol' cheesegrater had a full strip-down, clean (which includes taking the cover off the PSU, they make excellent dust filters!) and some fresh thermal re-paste?