A: This reboot is by design. To enable the Windows 8 touch-enabled boot menu, quite a lot of code has to be loaded to enable the various touch drivers to be loaded to enable touch selections of the required OS.
If an OS other than Windows 8 is chosen, then this loaded code is no longer needed. Instead, the other OS boot code must be loaded so the boot choices are stored and the machine rebooted to start into the chosen OS.
Read more FAQs on all things Windows--and beyond--at John Savill's FAQs.