Why does my MBP say it was restarted "because of a problem"?
I had the same annoying issue on OSX Mavericks (10.9.2), and resetting the NVRAM / PRAM as per Andrew Ferrier's answer didn't resolve it.

Instead, I opened a Terminal window and found a week old Kernel .panic file in /Library/Logs/DiagnosticReports/

I simply renamed it to .panic.old with the following command:

sudo mv Kernel_[rest_of_filename].panic Kernel_[rest_of_filename].panic.old


After a reboot I no longer get the annoying message!