And these rehearsals, even the ones leading up to dress rehearsal and taping, are probably not "Okay, lets mark through the choreography, run it once or twice and we'll be done" rehearsals. I'd venture to say the pressure to dance full out, 110% the entire time, even in a rehearsal that's only an hour long, can be exhausting, doing that 4, 5, 6 days a week, and well, this is what happens. I seriously doubt the dancers are marking anything in rehearsals at all.
I'd venture to say that, for the most part, these rehearsals are probably more intense than most professional or pre-professional or studio companies out there.
And it's not just contestants getting injured either, we've had All Star casualties as well.
I'd also be interested to know what kind of warm ups, if any, are given in these rehearsals, or if the dancers are responsible for warming themselves up prior to rehearsal. If they are responsible for warming themselves up, I'll bet some are going into rehearsals cold or virtually cold, at some points, and that could account for injuries as well.