Tumbling is caused by a loss of stabilization or disruption of the bullet, while the bullet is in flight. What some believe is tumbling, often indicated by a keyhole or side way shape/hole on the paper target is many times, the target tears from the energy of the bullet.
So unless something was causing that Shockwave not to spin properly, I can't see how it tumbled. Now stabilization is effected by many things.. the amount of energy behind the bullet, the twist of the barrel in correlation to the projectile being shot, and possible deformities in either the bullet or the sabot.
Since it happened on the third shot, my first question would be, was he swabbing the barrel between shots? My next question is, has any other bullet done this to him? And last is, how old is the powder?