The discussion about whether switching disciplines increases or decreases injury risk is a bit abstract without at least rudimentarily addressing what the switches imply position-wise (which is still a far cry from addressing cases individually, of course). Generally, an MTB will have a more upright position than a road bike, which usually means quite a bit less anterior pelvic tilt and hence a more rounded back position. MVDP does not have the flattest of lower backs on the road bike, either, but I would bet it is closer to his neutral position than his MTB is. Now, add repeated & steep climbs done at high power outputs but often at lower than road cadences (meaning higher torque), and I think it is not outlandish to suggest that an overuse injury to the lower back may occur - especially if the rider in question is in any way prone to develop back problems.
Many will of course get away with the switches no probs, too, but I would guess the pro level power outputs are a risk factor even if their core strength is top notch.