"The length of time in stalls may have an influence."
I'd be far more interested in a study where they looked at a population of horses that had turnout and normal social interaction with other horses. The fact that 65 out of 76 horses have stereotypies suggests that spending 23 hours per day in a stall is the principle cause of equine stress and neuroses and subsequently, developing behaviours like cribbing, weaving, and so on. Then I'd argue that the personality of the horse, the type of work it's asked to do, the diet it's on, who it's neighbours are, and other variables come into play in determining whether or not an individual horse housed under those circumstances acquires a stereotypey behaviour. I think this article (and possibly the researchers) were too quick to link it to the demands of upper level dressage. It might be, but it might not be. Here's a theory I think equally as valid. "Hot" or high-strung horses kept confined most of the day may well be more likely to develop these behaviours (in my experience, they are). It also so happens that most upper level dressage horses are fairly hot, while a vaulting horse is generally a laid back and relaxed character. So the type of horse more likely to succeed in the upper levels of dressage is also the horse more likely to acquire stereotypies in certain kinds of environments.
The second article on the other hand offers some solid empirical evidence that social isolation can cause stress related behaviour.