Consider this: When you're carrying a payload in the bed, of say 1,000 lbs (1/2 ton), the center of gravity of that load is somewhere in the bed, usually between the front of the bed and the rear axle. That center of gravity is then between the axles, and it helps with stability by way of increasing the load on all 4 wheels.
Now, toss out that load, and add a trailer with a tongue weigh of 1,000 lbs. That puts a 1,000 lb load on the trailer hitch, which is behind the rear axle. The rear axle then acts like a fulcrum, and instead of having some of the weigh pushing down on the front end, you have leverage pushing upwards.
That, in and of itself, is a lot of cause for twitchy feeling steering when towing. You may have the tires pressurized correctly for the best driving control, but now you've changed that by decreasing the load on the front, which will allow the tires to sit up higher, and stiffer, not exactly what you wanted. Depending on the load and the trailer hitch, you might actually want to reduce the load on the frone and to get the desired results.
