Many things cause subluxations. I feel a lot of trials working may be too much for dogs but they are so willing to please that they do it anyway. Twisting between poles, jumping, etc. also put quite a strain on the back. Similarly with 'man work' the dog may get carried away and can really put a great deal of effort into its work. In my opinion, care should be taken in getting the dog as fit as possible before work.
Fitness includes suppleness as well as strength.
There may also be grounds for discussing the present standards of equipment and whether they need to be altered to cause less strenuous contortions.