The brake pad hitting the sidewall is the most likely culprit. When the brakes are applied and they will bend and rotate counterclockwise, so the top-front of the pad would be the most likely to make contact.
That could happen if it were a front brake, but rear brakes arms, being behind and below the mouting bolt, tend to bend downward as they bend forward.
But maybe there is another brake actuation dynamic at work here. On dual pivot brakes, the right arm on the rear brake rotates on a offset pivot. Because the pivot is offset further form the center than the pad/rim contact point, when the brake is actuated the right pad will move in an arc inward and upward. As the pad wears, the arm moves even further in the inward/upward arc, and if the pad worn down too much the shoe may actually arc upward far enough contact the tire.
So, I guess the questions are: Are the pads worn down on the brakes? When the brake is actuated, does the brake shoes arc upward enough to contact the tire?