I think you've got flexure in your optical train. Make sure that there is no slop in your focuser. Check that your scope is balanced on your mount. Your camera is suddenly shifting orientation. If unsure make sure that everything is locked down. Grab your camera with one hand and put some force on it in all directions. Is there a clunking sound or can you feel something not right?
That certainly is a possibility, field rotation with an alt/az, but I would expect something different looking. This looks like a sudden shift. Let's let our alt/az astrophotographers chime in.
The good thing is that if it is entirely due to alt/az, you can crop the image.
this effect is a result of the mount being alt/az, so as it tracks the sky the view seen by the camera rotates slowly. I am assuming that you were live stacking in SharpCap - the live stacking corrects for the rotation, lining up the new images with the original one, but it cannot do anything about the bits of sky that were in view to begin with but are now out-of-view due to the rotation. Those out-of-view areas receive no new data and gradually darken as the stack progresses.
Nothing really to worry about - just be aware that if you image things near the zenith (overhead) then the rotation is quicker than if you image lower down in the sky. At the end of the process, crop the final image to remove the dark areas.
