Based on conversations with biologists and wardens, one of the contributing factors is prairie dog shooting. Millions of rounds expended each year, where the carcass is left behind entirely for scavengers. That's a lot of lead in the blood streams of raptors.
When I was with FWS, I captured a golden eagle near pdog town with lead poisoning. It survived after a trip to the Recovery Center. Who knows how many just die out there.