We still don't know the trailer model.
When boat trailers track with 5% load on tongue just fine, typical cargo trailer will be prone to fishtailing with this weight distribution.
How sensitive the subject can be, on the mentioned cargo trailer I was carrying small bulldozers.
When I load it with plow forward - the trailer drove rock solid.
When I loaded it with plow to the rear - it become wind and road crown sensitive.
I observe to have load COG at right place all the time, so the difference was in air drag.