Does your trailer only have hydraulic brakes?I put bucket out by side and let it down slowly but I see a driver of mine going up just using his pedals and then letting it down of its own accord. I can’t stand watching it as I feel he has no control and besides I prefer one hand on levers and one to control the boom for that operation. I guess what I don’t like about my trailer is that I overhand 3 inches each side, and I feel the trailer does a lot of shaking and lurching and is not stable while been loaded. Is a triple axel a better job? Also the wheels are recessed an inch or two under the chassis and I feel that contributes to instability.
Having the trailer brakes on while loading makes a difference, if you have no air brakes I would suggest making an adapter so when you go to load excavator, uncouple the brakes and connect to a spool and pressurise them.