Weight behind the axles of your trailer tends to cause or aggravate sway.
The more weight and the further behind the axles, the worse.
Our trailer has a large rear trunk in which the previous owner stored his
trailer dolly, which is a really heavy hunk of steel. Spare tire, jacks,
spare parts, etc., are also in the trunk, and our two bikes are on a rear
bumper carrier also. Not good but we get away with it. We have never had
any sway whatever even with no sway control (forgot to tighten it) but I
do put the dolly up front in the living room.