Final answer:
The common causes of a trailer jackknife include oversteering or overbraking, and if a trailer bounces up and down slowly on the highway, it is likely nearly empty as a heavier load would stabilize the trailer's movement.
Step-by-step explanation:
The most common causes of a trailer jackknife are oversteering or overbraking. When a driver oversteers, they turn the wheels more sharply than necessary which can cause the trailer to swing out and potentially lead to a jackknife situation. On the other hand, overbraking can cause the wheels of the trailer to lock up, losing traction and creating a situation where the trailer swings out uncontrollably in relation to the cab, also leading to a jackknife.
Both of these scenarios are more likely when road conditions are poor, such as in wet or icy conditions on the highway.
If you notice a trailer bouncing up and down slowly on the highway, it is more likely that the trailer is nearly empty. A heavily loaded trailer would have less bounce due to the weight stabilizing the trailer's movement. An empty or lightly loaded trailer has less weight to dampen road bumps and vibrations, which is why it may bounce more noticeably.