It has to do with torque. Think if you had a bike wheel spinning freely. Is it easier to stop it in close near the hub or further out near the tire. The further out you are the more leverage you have.
Now apply that to skis. If you ski length is staying the same and you lower your bsl you are essentially making that hub smaller. The same force at the tip of your ski will have more leverage because the lever (your ski) is now longer. This means you have to increase din to have the same retention as before. Same thing happens if you keep bsl the same and lengthen your skis.
Science lesson over. Hope that helped.