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You probably accidentally rebooted it in fastboot (bootloader) mode by having the volume buttons pressed when you turned it on. It's harmless
PS: this is surprisingly easy to do unintentionally, depending on how you grip the phone.

I can imagine. At least you didn't press any buttons before rebooting - had you got into recovery mode (android lying "dead" on its back with big exclamation mark above it) you really would have known panic if you didn't know what caused it![]()

