This is happening to me, too. In my case I will be in desktop mode and it will randomly switch to tablet mode. It does seem to happen more often when I launch a PC game that uses full-screen mode. Note that whenever I launch that PC game the monitor will always blank out for a few seconds with the message "no signal detected" before it comes back with the game screen.
It also sometimes happens when I first start up the PC.
I have the Surface Book, the Surface dock, a USB keyboard (to the dock), the Arc Mouse (Bluetooth) and an external monitor connected via DisplayPort-HDMI dongle (from Microsoft) to the dock. I also have a Jabra Bluetooth headset plugged into the dock for USB charging.
I've set feedback to automatic and diagnostics to full in the hope that Microsoft will get the data it needs to fix this. Hopefully that will arrive soon. It is very annoying.