It has now happened twice in under 12 hours. Both times I was able to re-jailbreak the phone without tethering.
The phone definitely did not lose power or reboot either time.
In my reading, this seems to be a "reboot" of unc0ver.
When I have a chance I am going to un-jailbreak, then uninstall and unc0ver, Cydia, and Flexispy.