I didn't find the instruction manual which mentions carrying too much, but I found another with recommended speeds.
I think the top speed on a BX is about 8mph, "full throttle, full pedal, high range," so if you weren't near that, you may still have been too slow.
From the Gearmore manual
Gearmore, Inc. - Grading Scrapers, GS/GSI Series - Grading Scrapers, GS/GSI Series
2. Start on the far right hand side of the road. Make your first pass at a slow speed, approximately 2 miles per hour. This will allow the unit to cut out and fill holes. Work the far right side on the road and return by grading the opposite far side of the road. Continue until you reach the center.
3. To finish grade, it is usually necessary to make one more set of passes at a higher speed; approximately 7 miles per hour. The higher speeds allow the material to flow over the first blade, thoroughly mix between the two blades, then exit from the rear blade achieving a smooth finished road surface.