I think the biggest contributors to getting PMR and GCA are genetics and age. If you have those two starting conditions, any stress or combination of stresses can trigger PMR and GCA. It could be long term overexercising, diseases like Covid, emotional stress, vaccines, etc.

It does seem like a lot of people that get PMR and GCA are fit and active when they get sick. It could be that along with their fitness they have a highly tuned immune system that malfunctions and starts attacking the body.