There are a couple of possibilities I can think of offhand regarding the deflectors cracking...
Firstly, given that they are held on by a single clamp on each corner, there is always the chance that the screws are too tight...The clamps don't sit flush against the body (as the corners of the moonroof are rounded and the clamps are flat), so depending on how tight you have the screws, the added pressure of the moonroof frequently opening and closing can add strain to them...If the cracking you're getting is a star shape around the screws themselves, or a line stemming from the screw hole, this is the problem for sure...
If the cracks are just randomly happening around the sides or corners, it could possibly be damage from automated car washes...So if you run your 5 through the wash every now and then and there are powered brushes doing the cleaning, they can always press too hard upon the roof of the car...
Hope this can help you solve the problem...And I'm sorry to hear about it cracking on you...Maybe you can get the dealership to replace it under some sort of manufacture's warranty???