How does it end up on it's back?

If there is too much right thrust it should yaw right, but this should not cause it to roll over, even with the built in dihedral.

From the all too brief description it sounds like the plane is rolling too, more so than you ( the pilot ) is able to compensate for.

Usually on a new plane, during the first flight, you trim the plane out so it does not roll, etc. in flight. Has this been done before by a more experienced flyer?