Ed,
That is weird behavior. I would start by checking the ConfigTargetOSHandler.log under SystemDrive\Windows\Temp while in WinPE before it reboots. See if there are any errors in there. It'll also be placed in this location on the Windows image if it reboots before you have a chance to look.
This document may prove useful as well: How to troubleshoot the Configure Target OS (CTOS) Action in Provisioning Templates
Thanks
Aaron