Why does removing the GPU reset the Bios?
And if my previous settings produced a stable overclock, can I just set them to what they were before and be done? Or do I still have to stress test again?
Removing the GPU usually doesn't reset the settings. If your computer is over 3 years old the button battery which powers the real time clock and SRAM that stores firmware settings on older PC BIOS motherboards (newer UEFI motherboards store them in non-volatile flash so pulling the battery will just muck up the clock) may be dying.
Some motherboards with buggy firmware may detect a firmware mismatch and force the settings to reset for stability purposes.
Losing system settings is an unfortunate fact of life for overclockers and enthusiasts. I'd rather they be reset occasionally than refuse to reset when something goes wrong. If you're not used to it by now you should be soon