Bald Eagles are ALWAYS out to prove just who is the best, strongest, top-of-the-heap.
Or, in this case, winner of the favored branch for a perch.
The 4-1/2-year-old eagle with its back to you has decided he wants the perch the other eagle has already landed on. Hah!
The velocity of the impact and gravity takes effect and the other 4-year-old eagle is knocked off the perch.
Then the chase is on as the one who started the affair decides he really didn’t want the perch; he just wanted to prove he was the stronger bird.