It's normal.
At idle you're at 26inHg of vaccum inside the manifold, and when you open the throttle, you have almost atmospheric pressure.
This difference of pressure added with the design of the manifold (distance between manifold mounting flange and plenum) makes the whole thing move.
That's why the guys that went with forced induction have to strap the manifolds together
And also why Porsche is using a plenum with spin-locks and tie bar on all the last generation 911 with turbos!
IMGP3367.jpg Photo by caseyparkin | Photobucket