I'm with bryson on this one. When I first got the revo, I broke 4 headers in 2 months. At $23 a piece, it added up pretty quickly. None of which where directly related to a rear landing. The all broke right where it bolts to the engine like yours. All of the broke upon landing a jump. Just the impact of the chassis slapping the ground snapped them.
The os 18TM revo header hasn't failed me yet. Considering it uses springs to hold it onto the engine, it allows flex from the spring, not from the header itself. Plus, OS's is just built better.
A few weeks ago, the same thing happened to my jato. In the exact same spot as it did on the revo. I replaced it with a spare (non traxxas) header I had laying around and haven't had a problem since.