It's created by what percentage of total vehicle/trailer weight is on each axle assembly. It's exaggerated by some long GN trailers that have frame sag under stress. Then add the flex of the tow vehicle springs under these conditions.
If the load isn't properly distributed and the vehicle hits a bump, or in extreme situations something so simple as the concrete cuts on the hiway, the vehicle spring movement, GN trailer frame sag and improper balance of the load cause the vehicle and especially the trailer to "bounce". This bounce is transmitted into the ball assembly on the tow vehicle. The forces there actually yank rearward and slam forward in rapid succession. Causes the "cab" including your driver's seat to slap you in the back. Very uncomfortable. A little unsafe. But primarily a Pain in the Back after only a few miles.
Moving the load a few inches can diminish or eliminate this absolutely undesirable characteristic.
Friend of mine has a 35+5 GN trailer that is horrible about this. Primarily due to frame flex in the trailer. To haul his Kubota CTL he has to park it centered on the trailer axles. In this particular case I totally blame the trailer. Poorly built!!!! Popular brand trailer.