I have the same problem - sometimes. I've only seen the problem on cold start. I suspect it's an issue with how quickly the PC boots vs. how quickly Start8 starts. When I just log out and back in after the PC is already started, it goes to the desktop every time. I did note that Task Manager shows Start8 as a high impact on start-up. I'm not sure, but it looks like W8 may be delaying the start of some apps to speed the screen display.
I've also noticed that the Start button for Start8 will not display (again sometimes but only with cold boot). As above, if I log out and back in the start button displays. The start buttons functions are all there even if it doesn't display. I just have to click on the 'empty' space.