Dual axle sway

The big enclosed trailer was made in the UK and the running gear was
from Germany. The big Tundra did a good job towing until about 60
MPH when the trailer would start to sway. I'll fix that...easy. I toss
in a couple of bags of crushed rocks in front. No change. Another bag
was added to the front for a total of 120 pounds as I recall.
Amazingly, there was no additional tongue weight. How can that be?
The sway was just as exciting.
I'll fix that! Now it has a new single axle and all is well.
GA