It looked like the eagle was about deep enough to be getting lung, so the pronghorn was probably delirious by then and literally on its last legs. When i watched a show about golden eagles, they were killing wolves by flying in and sinking their talons into the lungs and then just waiting til they bled out. I bet that's what happened there (2 holes in its upper chest), and the pronghorn was just bleeding out, and the eagle didn't see it as a threat so it started early. Nature is brutal.