Birds of Prey Harassed or Mobbed - Why?

I was driving home this afternoon in northern California and saw a large bird soaring fairly low over a very large grassy field off the road. I thought at first it was a Turkey Vulture, as they are abundant in our area, but I noticed it had a definite white tail with a wide black band. I had to pull off the side of the road and watch it for awhile to make sure I could identify it once I got home.