Out of balance posture to the glove side.
Weak back side with little or no rotational torque.
He needs to keep his head over his hips, when your head is outside of your center mass, it takes energy to maintain your balance that could be going into the pitch and traveling toward the plate instead of traveling toward 3rd base to offset the head being on the 1B side.
He needs to work on extension off the posting leg. You see how his posting knee comes straight forward after release? That's a sign that he's not getting a lot of torque and possibly a strength issue with his glutes vs his hip flexors. His posting leg should follow through in an arc due to the rotational torque of his upper half. As he is now, he has no rotational energy or upper half separation and his flexors are overpowering his glutes and allowing his back leg to come forward.