In case anyone else comes across this.

If you are using WinPE to self boot BurnInTest for hardware testing and then are running long tests, you will see automatic reboots at the 72 hour mark.

This is a limitation of WinPE
"To prevent its use as a production operating system, Windows PE automatically stops running the shell and restarts after 72 hours of continuous use. This period is not configurable."

The time limit is different with different versions of WInPE
24 hours - Microsoft® Windows PE, Version 2005
72 hours - Microsoft® Windows PE, Version 2.0, 2.1 and 3.0

This doesn't stop you doing multiple runs of 72h if you need a really really long test.