Yes, bulls will rub for so many reasons in the rut & out of the rut. From rubbing velvet off to displaying for one another late summer in bachelor groups getting their pecking orders down in prep of the oncoming rut. They can feel their oats as their testosterone levels rise as Coveyleader mentions & can take their excitement & aggression out on trees & brush or whatever is around, they will also do this at wallows wet or dry, all this during & awaiting cows to come into estrus.
Bulls raking & thrashing around at these times are not challenging other bulls. As cows enter estrus in their harems you will see herd bulls become very vocal with more intensity to their bugles as well as being defensive towards other bulls getting too close to his hot cow. A herd bull can bugle, grunt, glunk, pant, rake, etc. at this time as he displays excitement over her & warn other bulls back or else! When no cows are in heat/estrus you will notice herd bulls will not employ these actions, they are more subdued & at times will allow other bulls to walk right among his cows without retaliation.
I've been in Brendans situation countless times & have messed a few up as well. (grin) You never know when a bull may slip in around you silent as a ghost! One thing that has helped me in similar situations is when using a raking method towards another bull give him a good reason to come your way. Yes, raking/thrashing employs curiosity in another bulls minds eye as to "who are you" but feel free to raise the anty here. This can be done by first cow calling with 2-3 mid volume excited mews/whines once in near that 100 yard range of a bull you've heard bugle. If you cannot close the distance by ninja moves to him then bring him to you. Use the same sequence or similar one he uses as he has a hot cow present.
If you cow call your 2-3 times & no answer wait 30-45 seconds & repeat, whether bull bugles you or not wait another 1 minute & start raking tree branches & light stomping, the real bull will hear this & realize this new bull is displaying for this cow. Give 1-2 more cow whines & now give 2-3 bull pants through your bugle tube along with more raking, (don't bugle yet) this shows excitement for the cow. If the real bull bugles this & he should you can now give him a challenge bugle back! This defensive action from you is now telling this real bull you have a hot cow. You may have to repeat this sequence to really trigger some bulls to come your way, they don't always bugle you but most show up so stay alert & listen & watch intently.
I have pulled the most stubborn bulls into bowrange with this simple technique on OTC DIY public land hunts. With a good setup & wind in your favor it's very deadly.
ElkNut1