At first I thought it was the plastic piece chaffing against the leather, which would mean the leather is moving vertically, but after looking at mine, it's pretty snug. I had to use some pressure to get my finger inbetween the leather and the plastic. I checked and the plastic piece on mine is smooth along that edge, so it wouldn't cause that kind of damage to the leather.
I think what's happening on your seat is the leather isn't moving vertically, it's bulging outward which is causing the surface to start cracking along the bulge crease. That's the kind of thing you usually only see on pleather (plastic leather), where the surface is different from what it's attached to so as it's stretched, the surface shows stress cracks like that. Real leather isn't going to do that unless it's very very dry which means it can't flex very well.
I would definitely bring this up to the dealer, but a temporary fix might be a good leather conditioner. Meguiars makes some good leather care products for cars.
http://www.meguiars.com