Final answer:
Loading cargo to the rear of a trailer creates an unbalance that makes the trailer difficult to control. A trailer bouncing slowly on a highway is likely nearly empty, as an empty trailer has more suspension travel and bounces more than a heavily loaded one.
Step-by-step explanation:
When loading a trailer, if the cargo is loaded all to the rear, it may result in the trailer becoming unbalanced and difficult to control. Placing heavy cargo at the back of the trailer can shift the center of gravity too far rearward, reducing the weight on the hitch. Such an imbalance can cause the trailer to sway or fishtail, particularly at higher speeds or when making turns, which makes it difficult to control. Moreover, it may increase the risk of the trailer tipping over if the weight causes the center of gravity to move outside the area of support. Conversely, a properly loaded trailer, with the weight distributed evenly or slightly towards the front, would be more stable and easier to tow.
Regarding the observation on the highway of a trailer bouncing up and down slowly, it is more likely that the trailer is nearly empty. A heavily loaded trailer would have less suspension travel due to the weight pressing down on the springs, leading to less bouncing. An empty or lightly loaded trailer has more suspension travel, hence it bounces more when it hits bumps or uneven road sections.