A non-sudden matter-of-fact move with a few words can help reduce the chances of that, especially if it's all done in a certain Notice ->BeginHelping / ShowYouCanHelp -> GiveAnEscapeRoute+OfferMoreHelp pattern.

If someone is carrying heavy bags and looks like they're about to fall a flight of stairs, I put up my arm to help and then ask if they want help, or some other signal of offer-for-assistance that gives some kind of opening to say "No thanks" or walk away.

If your actions and/or words pattern-match to this kind of intervention, then the vast majority of people will look positively upon it, IME.