The trailer is too small and the wheels can act like a fulcrum if the load is not balanced. The Suburban - at 2640kg - is likely heaviest at the nose. The E39 estate's towing capacity is a mere 750kg unbraked and 1800kg braked.
When decelerating hard - the trailers load could potentially shift forwards and put undue strain on the ball hitch. The hitch - now loaded more than what it would experience than by simply towing - could either physically break it's mount to the E39 - or cause the E39 to pitch upward using the rear axel as a fulcrum - thereby reducing its steering ability.