A kettlebell swing should not be a squat. If the kettlebell travels below the knees during the exercise then you're squatting your kettlebell swing and putting too much pressure on the lower back instead of using the glutes.

To correct this, stay upright a lot longer than you think before you start hinging (or bending) at the hips. One way to get yourself to hinge back is to imagine the movement you'd make if you were kicked in the junk. Your hips are going to hinge back violently – you're not going to do a squat.

Related:  Kettlebell Swings – You're Doing Them Wrong

Related:  Take Kettlebell Swings to the Next Level