Was watching a couple of Youtube videos of some cars going in a straight line and braking heavily and then losing control and back of the car spinning round.

Just wondering what is causing this as looks pretty dangerous and to be fair I have had a little shakey-ness before under firm braking that I didnt like the feel of.

What can be done to eliminate this, or is it just the physics?

if the rears brake too much you can invoke oversteer

toe out on the rears also results in oversteer

It can also be over powerful front brakes giving a sudden and massive weight transfer.

It's why modifying can become a slippery slope. You change the brakes from standard, but then your suspension may need addressing which may lead to issues with traction and so on.

The only sure way to stop it is by driving slower

I had very bad squirming under heavy braking in my car. I overhauled the rear brakes and the compensator and the braking is now straight and true under hard braking. The rear brakes were adding virtually nothing to the braking effort!

The combination of good brakes and bigger rear ARB may cause this yes. Oversteer. We just have to cope with it..