You have nothing to worry about. You are just getting very good traction, and using all the power you have available at the RPM you are using, and the range you are in. Unlike a gear drive transmission (where the tractor would stall out in a similar situation) you are just stalling the transmission by exceeding the pressure the bypass valve is set at. As others have stated, keeping it in "turtle", and working at the rated rpm (3000, or near wide open throttle) might change that. The BX series is designed to work at high rpm, and will give much less performance if you drop the rpm much.