I had the same problem with my X-Vision. It was about a year and a half old with about 30 dives on it. I noticed cracks in the frame when I was getting ready to dive. Just like you, I pulled on the cracked piece of frame, and it crumbled. The plastic appeared to be brittle. Luckily, someone on the boat had an extra mask, or I would have been watching bubbles.
The mask was in mint condition. Always kept in the box, and cleaned well after each use. Not one single impact mark or sign of damage causing the cracking.
There was a previous post about this problem with the X-vision last month. The post had pictures of the failure in his mask, that was identical to mine.
I took the mask back to Divers Direct in West Palm Beach. First they said it was out of warranty, and they could not help me. When I went to their display area to look for a new mask, I found that their display X-vision had the same small craks forming on it. When the manager saw this they gave me a new X-vision.
I love the way the mask fits, but now I am constantly worrying about when it will fail again. I am now looking for a new mask, possibly an Aqualung Mythos...