Honestly, there is no one thing to look at that would indicate why a box rebooted.
The good news is that the information is usually contained inside of a "Tech Support" file that you can gather from the WebUI.
The bad news is that Support would need to look at that file to determine why the reboot happened.
Please open up a support case by logging into support.paloaltonetworks.com or calling in at +1-866-898-9087, gather a Tech Support file from the device, and then we should be able to tell why it happened.
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