It never hurts to have a chiropractor or other sort of back specialist out anyway... in my experience a lot of horses have at least minor back problems when you buy them. Then if there is a problem you can deal with it.
It can be a lazy thing or a learned behaviour. Some horses trip and then the rider stops "riding", lets them have their head, lets them slow down etc and they learn that tripping gets them out of work.
Have you tried loose lunging him in a round yard, keeping him at a good trot for a while? Does he keep tripping or does it reduce when he warms up? Does he trip in the paddock?