As it happened to me, i noticed the trailer swaying in my mirrors , then i felt that force pushing my hitch which made the steering feel lighter.
All three forces seem to occur simultaneously. As I let off the gas, the trailer fell back in sync. It was a small trailer at about 2000 lbs with no trailer brakes . If I had brakes on the trailer I would have let off the gas and applied brake pressure to the trailer brakes. Its usually NOT a good idea to speed up or use the brakes on the tow vehicle .
If you have not had any sway yet, then your trailer is in balance or loaded correctly. Therefore (besides a flat tire or other mechanical occurrence) strong crosswinds or a bow wave from an overtaking truck or bus would probably be the reason why your trailer would sway.
