Bucks make rubs after the antler hardens and while velvet is shedding, but from what I've read, they are not rubbing to intentionally shed the velvet, or to build neck muscle.

Rather, it is the beginning of the time of year when testosterone is building up and the bucks are just looking for something to take out a little aggression on. When you see a concentration of rubs that are not in a line, it suggests that the deer is spending some time in that area, and often that area is a bedding site. Not always of course. And, it's not like you can ask a deer what he's "thinking".