I think Phil has been instructed by doctors to wear protective headware from the sun. If there's any risk for skin cancer, that is often what patients are told by their doctors.
Until the last year or so, Phil may have had headwear on once or twice, but it looks as if the is wearing the summer hat whenever he has to wait for teams in direct sunlight. If its a location where he's not in direct sunlight, then he seems to not wear headgear.