Why did Buck charge at "Black" Burton's throat?

Buck charged at Burton's throat because he struck out at and knocked Thornton over, almost causing him to fall to the ground.

“Black” Burton, a man evil-tempered and malicious, had been picking a quarrel with a tenderfoot at the bar, when Thornton stepped good-naturedly between. Buck, as was his custom, was lying in a corner, head on paws, watching his master’s every action. Burton struck out, without warning, straight from the shoulder. Thornton was sent spinning, and saved himself from falling only by clutching the rail of the bar.