If you haul loads often you have experienced trailer oscillation. Which is when the trailer and vehicle begin to move back and forth in opposite directions around a pivot point, usually the ball. The load may feel fine but out of nowhere it begins to move uncontrollably.  I have had this happen before and it is unnerving. In most cases it is due to the loading, specifically to much of the load on the rear of the trailer. Once the conditions are set by the improper loading, all you need is a dynamic force such as an abrupt lane change to set the oscillation in motion. Now days we spend a lot of time ensuring the load is forward of the axel and test drive the load prior to delivery and make adjustments as necessary to ensure a safe, uneventful trip.

Take a look at the following video, it makes the problem easy to visualize.