I would say my general rule when a horse moves and I haven't asked it to, is to make it move a lot more than it wanted to. Get the feet moving in any direction; backing is just usually the easiest. If a horse is backing, side-stepping, rearing, etc, then the horse's attention needs to be moved and the rear end disengaged by tight circles until the butt moves over, or what have you.
Of course not every situation is the same, even with the same horse. The point is to make the wrong thing hard and the right thing easy, and always make sure the focus is on what the rider wants it to be on.
Also, tack fit is a huge one. If a saddle is ill-fitting, a horse will not be happy. Likewise, a rider trying to heave themselves up from the ground when horse/rider/both are unbalanced can lead to struggles. Evaluating the individual situation is the best option IMO.
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ETA: Also, it shouldn't be teaching the horse to back up as you would only be doing this IF they moved, and ideally doing it right away so the horse makes the connection.