why mad cow?

Because the cows started to behave madly (or irrationally / strangely if you prefer)

That's why. It's also easire to remember than Bovine Spongifom Encelep... whatever it was

mad cow disease is a chronic, degenerative disorder affecting the central nervous system of cattle.

It is called mad cow disease because it affects a cow's nervous system, causing the animal to act strangely and lose control of its ability to do normal things, such as walk.

Bovine’ refers to cows; ‘Spongiform’ describes the ’sponge-like’ effect of the disease on cells; and ‘Encephalopathy’ is used to describe diseases of the brain. While members of the media often use the term “mad cow disease”, the correct name is Bovine Spongiform Encephalopathy, or BSE for short.

The disease is only caught by bovines which generally means, in the West, cows (we don't have so many water buffalo, for e.g.)

The disease attacks their brain and makes them insane, or mad.

So "mad cow" - exactly what it says!