That loader adds quite a bit of weight above the axles, thus making it a bit less stable (raising the Center of Gravity).
Unless I need the loader, it's off the tractor...just because of that reason.
So, as our friends have said, you need to add weight. Where you add is is important. Filling the rear tires is, IMHO, the best way to go. Rear weights, such as a weight box or implement is good too, as long as you keep them as low as possible.
And, of course, technique is important. The way you operate your tractor with the loader is different then when it's off. Definitely time to slow down. Going across slopes should be avoided if possible. Going across slopes with the bucket raised is a definite No-No!
If you think your jobs out, you may find a safer approach (straight up and down slopes is best) to do the job.
And, I assume you do have your seat belt on...right?
This is the first post I've read on the BX series lacking stability. To me, that's one of this little tractor's prime selling points.
