As was stated before, many possible causes, but I'll share my story, there may be some value in many perspectives.
As weird as it may sound, I recently made the switch to shorter crank arms and find myself not riding so forward on the saddle. My sit bones are more engaged and I haven't tended hang my hip off the side anymore. It felt "weird" the first ride to be engaging my left quads and glute more and my hamstrings less. My hamstring action and compensating with my hip abductors in my pedal stroke was causing me to pull forward on the saddle resulting in "soft tissue pressure" if I didn't make a conscious effort to reset myself on the saddle once in a while. The doctor said left glute activation (or lack there of) was the cause and sent me to physical therapy. It always worked until I got back on the nike.
It would seem that the proper seat height for one leg isn't the same height as for the other. My legs became differently strong and hip ligament pain is where it manifested. I've been to the doctor several times, got MRI's, been through physical therapy, the whole bit..& it was over 2&1/2 millimetres of cramk arm length. Food for thought.
My left hip still hurts like a mo-fo, but this week it's the best it's been in a long time.
Here's a link to Mondays ride (the first with the new shorter crank) with comments to the effect of "We'll see how this goes." As you can gather here, I think it went well.
https://www.strava.com/activities/91...S&v=1490662952