What exactly happens behind a BSOD?

I remember the days of BSOD (Blue Screen of Death). You'd plug in new hardware and driver - and zoom the BSOD would appear on screen.

Can someone explain what goes behind the screens of BSOD?

I think it is due to memory dump.

For some reason, if the CPU usage of the system goes to 100 and stays so for a while,
then the CPU cannot perform any other regular functions to keep the system running.
At those times, the system restarts, and before that, the blue screen appears.

Usually an error message like "physical memory dump" will accompany the screen.

When working in a Windows operating system, there is a core piece of software that is designed to control all of the hardware operation within the system called the "Kernel". When the Kernel finds a problem in the system it will shut down the operation of the hardware to a bare minimum and take a picture of the exact location that it found to be the problem. It then displays this error information on a pre-defined "Blue Screen". At this point, the system is not usable because the Kernel has shut operations down to avoid further damage. Blue Screen Errors also known as STOP errors can be caused from any of the following:

Bad memory module
Poorly written device drivers
Corrupt Windows Registry
Incompatible DLL File