Did you reboot the computer a second time to see if it flashed the BIOS again? Once is a scheduled BIOS update, possibly triggered by an administrator. Twice is more concerning...
In theory there should be a setting within the BIOS that should allow you to disable any sort of automatic BIOS updating. Unfortunately, I can't speak to Dell machines, but the HP ones I manage have a "Lock BIOS Version" setting I can use.