I've had it a couple times. Hard to say exactly what brought it on. Probably too much of some one thing. The cure which worked for me was to walk with a weird gait where no one who knew me could see. As I walked, I'd rotate each hip is a circular pattern with each step. The first mile hurt like the devil but then the pain gradually went away, so I'd walk like 2 miles.

A few days of that and it went away, didn't come back for a long time. A riding buddy of mine tried that, worked for him too, so not totally nuts. It seemed to me that it was the Piriformis syndrome thing, which led me to do what I did. My WAG is that it was too much riding, not enough other work. Plus age, duh.