This is happening because you are using the dock with it’s attached monitor as the only display. There is a known bug with the Intel graphics driver documented on our product page for the Pro8.
"Problem: When I set my external display to be duplicated (cloned/mirrored), the setting errors out and will not let me select duplication, my external display is rotated incorrectly and the tablet resolution may be incorrect, or when using the external display only with the tablet display disabled, the screen may randomly rotate when opening full-screen applications.
Solution: Only extended desktop mode is supported at this time. Why? Due to Intel Graphics driver limitations for the Intel BayTrail platform, duplication and external monitor only configurations are not available over any USB DisplayLink graphics adapters like the Pro8 dock. These issues are subject to change with new driver releases and we will update our FAQs accordingly."
If you enable the tablet’s built in display and run both screens in extended desktop mode, this issue will go away.