why was Billee and Buck driven from camp?

"Joe whirled around on his heels to face him, mane bristling, ears laid back, lips writhing and snarling, jaws clipping together as fast as he could snap, and eyes diabolically gleaming--the incarnation of belligerent fear. So terrible was his appearance that Spitz was forced to forego disciplining him; but to cover his own discomfiture he turned upon the inoffensive and wailing Billee and drove him to the confines of the camp." Chapter 2

I don't think Buck was driven out though.