The rule of thumb to avoid trailer sway is to have between 10 and 15 per cent of the trailer weight
on the tongue. The reason for this is to put the larger amount of weight between the TT axel and the connection to the TV. This lets the TV have the maximum control over where that weight is going to go along with the back half of the trailer. The TV is able to lead the trailer into following directly behind it's direction of travel.
Given a 2000 lb TT, then Tongue weight (TW) should be between 200 and 300 lbs.
When less than 10 percent of the trailer weight is on the tongue things change. One of the reasons for the 10 to 15 percent rule is because of wind resistance. Wind resistance on the TT unloads weight from the tongue the faster it is pulled. That is why a trailer can be towed a slower speeds and not sway.
When the greater amount of weight is behind the axel then the tail can swing the dog. If you start out with 100 lbs TW and the wind resistance at 55 MPH is 150 lbs of force (just guessing on the number, but that is a reasonable number) , then the effective TW is minus 50 lbs. (the actual angle of the force is very complicated to figure out, but it is to the rear and downward. The faster you go the more force is generated and it's leverage unloads the tongue weight.) That means the rear of the trailer at higher speeds can out weigh the front of the trailer. When those conditions occur, when the back wiggles the front will waggle.
Wind resistance isn't the only factor involved, but is frequently not recognized.
Another factor is the distribution of the trailer weight. If weight is centered around the axel, then any weight added to the rear is going to have increased leverage because there is no offsetting weight on the front end that is countering the weight added to the rear.
The trailers center of gravity in relation to any added weight will also paly into a trailers tendancy to sway. If you have an air conditioner on the roof, then that will raise the trailers center of gravity and make the trailer more likely to sway if tongue is light
It gets complicated quickly when playing with the laws of physics. Leverage, forces, mass, momentum, vectors, and fulcrum interact in complex but predictable ways.