The toe is at a good length compared to the sole, yet the quarters are WAY longer than the sole when we look across the bottom of this foot. They are flared. So when the horse hits the ground, the quarters are taking alot more impact and pressure than the toe area. (imbalance) The pressure at the quarters with the toe floating over the ground are pulling the whole wall out on the sides like a paper cup being pulled apart by the sides, finally the wall has cracked to releve this pull and allow the foot to flex more and touch the ground level as it can.
Lower the wall at the quarters to the same length of the wall at the toe, keep it here, and the cracks will grow out in time when they are no longer being pulled apart from imbalance. This is a trimming error.