Sometimes when you see a bull off by himself, he may have been run off by a young bull trying to establish himself within the herd. The solitary isolation doesn’t last very long, because older bulls will band together in order to ward off predators, such as wolves, coyote, mountain lions, etc.
If a very senior bull has been run off by a stronger, younger bull, who is trying to establish himself within the herd, the older statesman will band with older bulls “kicked” out of the herd.