It's because the rear springs are too soft relative to the fronts you are using, and the additional weight from the Exotec is (mostly) in the front half of the chassis, effectively lowering the wheel frequency of the front. You could do the same thing with a softer spring and less weight (plus the benefits of less weight).

I had ten ounces of ballast in my truck (most of it in the front) at one time, plus a 550 motor that was 3 ounces heavier than the 540 I have in right now. Now that I understand that the springs are far too soft in the rear (but the stock fronts are okay) I can sky it out with the best of them without fear.

Cameron's truck is on a diet, nobody else should be afraid of the diet if they know what they are doing with shocks and springs. I know, big assumption there.