It definitely is related to how 8.1 attempts to detect and treats differently tablets/touch screen computers. I'm working with two different touchscreen laptops, one that does it and another that works fine. Its the smaller screened of the two that has the issue. however if I plug in a USB keyboard it finishes fine. As I understand it the presence of a keyboard is one of the triggers that causes 8.1 to go straight to desktop at login. What I can't explain is why the built in keyboard isn't good enough for the smaller model
I'm using sccm now but saw similar behavior with just MDT too. If I enabled the hideshell option it would hang for exactly 30 minutes "getting ready" but the ts was still running the whole time with the progress bar just hidden behind everything. So it like the os is waiting for the program to exit before you get past getting ready and when it doesn't it eventually timeout after 30 minutes.