What can cause a heavy trailer to fishtail?

A heavy trailer can fishtail due to several specific factors that affect its stability and control. When there are sudden steering movements, it can cause the trailer to sway, particularly if it's coupled with improper weight distribution or overloading. Overloading affects how much weight is distributed over the trailer's axles and can lead to instability, while improper weight distribution can create a situation where the center of gravity is too high or misplaced, making the trailer more prone to swaying.

Sudden steering movements can quickly shift the weight balance of the trailer, exacerbating any existing instability and leading to fishtailing. This issue is particularly prevalent with heavy trailers, where the dynamics are different from lighter loads, making it essential to maintain proper loading practices and operate with smooth driving techniques to avoid these situations.

The other options do not directly contribute to the fishtailing phenomenon. Underloading the trailer or smooth highway driving typically would not cause such instability. Similarly, using a heavier towing vehicle might help in controlling a trailer, but it does not inherently prevent or cause fishtailing.