I use a two horse slant trailer now and I do turn my horses (technically 1/2 turn) and lead them out - that is my preference for a few reasons. Usually I tie them for the trip.
With my first trailer (a two horse straight load with ramp added later; around 6 ft wide), I officially only had the option of backing them off. I had one gelding, however, (14.2; very stocky) who was able to turn himself around. It wasn't graceful and he wasn't one to panic so he could make it work; I believe a less confident or non-thinking horse would have got itself into a real jam trying it.
My point in this in an effort to respond to your question is: yes, a horse could get injured as you describe (or in another thousand other ways). It is most likely to get injured if it is inexperienced, panics or if there is handler error. If you address these areas through training of the horse and planning on your part, you will maximize the likelihood of successful loading/unloading each time you use a trailer.