He might have been demoted to being the 2nd and the other bull took over the position of herd boss. I've found many older bulls who get kicked off the main position withdraw and go off by themselves.
That could be one, but if he is sick or injured he would do the same.
If it were my place I would ship the older one bye bye and get a yearling as replacement. Prices are good right now, here at least, you might get just about enough money for the old one to buy a yearling, depending on your 'wants' in a new bull.
Agree with others, you definitely need 2 bulls for that amount of cows.
Have a good one.