When the truck is hauling the trailer there is approximately 10,000 pounds (actually more) pushing down on the rear axles. This adds more weight pushing the tires down, thus giving more traction. Without that added weight there is hardly any weight over the rear axles. Most of a truck’s weight is in the front (the engine, cab, passengers, and even the body of the truck), but virtually none on the back.
So why does this cause a problem? Well just like the small pick-up truck without sand in the bed, the truck won’t handle any better than a bicycle with only front brakes when it comes to sudden stops. The backend will swerve and slide, basically have a mind all its own.
Think about it, if you stop suddenly all the momentum is pulling you forward, without a trailer what is there to hold the backend down? I am not saying the entire truck will flip, but the back will raise causing less surface area of the tire to touch the actual ground. Not only will this increase you stopping time but you could lose control of the vehicle (much like the bicycle rider going downhill with only front brakes.)
