Why do eagles grunt like pigs?

I came home from work on a recent Friday afternoon to find a very large bird hiding among the bushes in my backyard. That’s not unusual. I frequently spot ravens, robins, jays and other smaller species bouncing around.
This particular bird would try to fly off, but it would fail to get off the ground and would return to very nearly the same spot every time. 
It would slowly walk around, sometimes up a hill into a wire fence. After emerging from behind the bushes, I realized that