We had it happen with just a couple of medium pallets going back on a MD-11. You have to have the nosegear strapped down or keep a loaded pallet in the front. Luckily the towbar was hooked on and kept the tail from contacting the ground.
The f/o onboard was trying to sleep, and suddenly got vertigo. He opened his eyes, and all he saw out of the windshield was stars!
Taxiing or flying the Mighty Dog empty requires 50,000# of ballast fuel. Any less and you're asking for trouble....