I wonder if it could be axle wrap due to those tall lift blocks and under slung axle.
Here's a common scenario: you hit a bump or pot hole and the component of the force that acts in the trailing direction (not all of the force is vertical) results in leverage on the tall block and spring over axle geometry which rotates the axle and cause axle wrap (the spring tries to warp into an "S" shape rather than compressing normally (flattening); then the spring snaps back slapping the tires into the ground violently (like wheel hop on a drive axle).
I realize it is not a drive axle, and someone is bound to argue that drop spindle axles don't have this problem, but there is a big difference in how arched leaf springs react to spring under vs. spring over with blocks.
Was it a large MC trailer intended for more than one full size street bike? 1750 lb rated springs sound stout to me for one bike, so I'm wondering how you were able to confirm the actual ratings. (Said another way, I would want to be pretty sure that I was not removing too much spring before I did it.)
Is the trailer at its final weight, or our you running around bare? (Sorry if this has been answered, I read the first part of this thread the other day and don't recall now.)
Spring ratings are not only dependent on the number of springs, they are also based on the length, width, material, heat treat, and, to a lesser degree, the amount of arch, so just because two different packs have the same number of leaves doesn't mean that the started out with the same rating.