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Not so much a rollover as the trailer tongue lifting the drawbar (say, going down a steep hill with a rear-balanced trailer full of gravel) and unloading the rear tractor tires, so you lose braking and control. The anti-uplift chain will pretty much stop that. The solid bars a poster mentioned will do the same but are not so strong in compression - mostly were used as anti-sway restraints, and to prevent the drawbar from falling.
Another point, irrelevant to some users, is the drawbar is strong enough to hold up most trailer tongue weights. However, you shouldn't pull a dead load from the middle of a drawbar unless it has the stabilizers other posters have linked to. I've seen drawbars wishboned when the pull spins the drawbar around so the strain is perpendicular to the width of the drawbar, and bends it in the weaker height dimension. Sorry, bad explanation, but you want the drawbar flat to the ground when pulling. Maybe someone else can make this clearer.
Jim