shear force from front squat

[REG]

Tom Kurz says, "The front squat puts more shearing force on the spine as compared to a back squat, but neither squat, if done correctly, produces enough force to cause injury to the back (Russell and Phillips 1989). The greater the forward lean in either squat, the greater the shearing force put on the back and the risk of injury."

I am confused, how can the front squat put more shearing force on the spine than the back squat, if during the front squat you are more upright? Wouldn't it actually be the opposite, in that, wouldn't the back squat put more shearing on the spine as compared to a front squat?