There is a bug in the viewer that occurs sometimes when you pan and zoom at the same time. I haven't found out how to reproduce it reliably, but try the following in IE11:
1. Zoom into an image.
2. Zoom out and pan (click and drag) at the same time.
Then the image is zoomed out but can still be dragged, which leads to strange effects. If you then disable Seadragon in the 42 menu it behaves normal, until you enable it again and the bug reappears.