I have had this problem with SCCM 2012 R2 as well. Particularly with the Optiplex 755 when trying to boot to Win PE 5 and run a Task Sequence. From the research I did, if either the mass storage or the NIC driver is not present, the system immediately reboots. For me, it was the SATA setting. We had it set to legacy when it needed to be set to AHCI. It apears Win PE 5 doesn't have the proper driver for "legacy" mode. You can troubleshoot this by enabling command support on the boot image (in the SCCM console, properties>customization tab, check the box). Then you can hit F8 when Win PE first boots and use the command prompt to check various settings (like whether ipconfig sees your NIC and you have a proper address). You can always add the drivers to the boot image in SCCM or simply start using Win PE 5. I haven't had to add any drivers to Win PE 5 for our 9020 machines.
