It is not normal, but it happens often.
The light path though the binoviewer displaces the focal plane by about 100mm. This means that any tilt in the system will be more pronounced because the focal plane has moved so far behind the telescope.
In Newtonians, this problem will show if the secondary centering is not perfect perfect perfect or and in refractors it can show if there is any sag in the system(often in the focuser due to the extreme load on standard Crayford pinion shafts) or of the diagonal is out of collimation.
In the SCT, most likely is either the diagonal is shifting in the visual back under the weight of the binoviewer, or the diagonal itself is may be out of collimation.
At any rate, it s not normal and is most likley something in the front of the Binoviewer, and I would look at the diagonal and mechanical connections first, but an out of center secondary mirror could be a contributor as well. To test this use a standard eyepiece and see if the secondary shadow shifts orientation as you move the scope though best focus. If the shadow shifts from one side of the Fresnel Pattern to the other, possible that there is some kind of tilt in the system.