Explanation

"When the trailer is loaded, the weight of the load presses the trailer tyres firmly onto the road surface, so improving the tyres' grip. Unladen, there is no load bearing down on the tyres, and under heavy braking the trailer wheels can lock or slide across the road surface, making the trailer more difficult to stop. Severe braking can result in jack-knifing: the trailer doesn't slow as quickly as the tractor unit and this causes it to pivot around the coupling."