hard for a ref to call over the back if there is zero contact to the opponent's back.

Definitely some contact in the left shoulder area as they both jump for it. I don't think that is a sign that SJ established box-out position on Kessler. SJ was ball-watching and jumped for it. Kessler runs to the side of SJ and jumps for it. The shoulder-to-shoulder contact is incidental.

i have not watched extensive film on BYU, but I watched several times yesterday and today that series where Pepperdine got 3 offensive rebounds in a row. At least in that sequence, SJ did not exhibit good rebounding fundamentals. On the first 2, he failed to put a body on anybody when he had the chance both times. Instead he ball watches as the shot go up and just jumps for the rebound. He almost got the first miss and then almost got the second miss, but did not. Rebounding fundamentals say to find the nearest opponent and box the guy out.