Something in the tow rig is loose or damaged. Also can happen if the toad itself has grossly worn or misaligned front end.
May be as simple as using a Drop/Lift Adapter, too much play where it connects to the coach then where tow bar connects to the Adapter, can add up to a wandering condition. Should NOT be more than half an inch of play, coach-to-towbar. From there, should be essentially none. I've towed vehicles on truck transfers and for repairs as well as RVing. It doesn't take much looseness for the toad to get crazy.