Just before the ball arrives, B48, who is standing very close to the receiver, jerks his head and shoulders up & back (note at 41 & 48 sec and 1:27 during the video). Perhaps the B, whose view of B48 was momentarily eclipsed by the punt receiver at those times, saw that and took it as contact between the receiver and defender.
I don't know Big 12's mechanics, but it appears that the B & S were fairly far behind the receiver. As the ball is coming down at ~B-30, the S is at the B-12. With his wider angle, if he were closer, he might of had a better look.
Geez, those two, huge play clocks at each end of the field are impressive.