I had a couple of bouts with this over the first two years post surgery. I had a real problem with it during the first month after surgery. After that, it was always brought on by the switch to my summer activities; tennis, surfing, and windsurfing. Dr Gross confirmed that my symptoms indicated trochanteric bursitis. He suggested light repetitive exercises. I found that this, along with use of the foam roller on the IT band, made it go away in a few days to a week. He also said that the next step would be a cortisone injection, but the roller and the light repetitive exercises have always done the trick for me.
For what its worth, I have a theory on what caused mine. Prior to surgery, my femoral head had worn its way well into my acetabulum, so my soft tissue had shortened to accommodate this. During surgery, he restored my normal acetabular depth, which lengthened my leg by a few millimeters. I think that the IT band has the hardest time dealing with the extra stretch, and it goes right across the bursa, putting pressure on it.