Why is the goat head associated with satanism? [Spirituality & Religion]

because goats are satanic

In the 1800s they misinterpreted the original meaning of a baphomet which would have just been another half animal, half human deity in the ancient world. It originally had nothing to do with satanism.
People fill in the blanks and create Chinese whispers

It's the horns

Sacrificial lamb... add horns and sacrificing something maybe for the devil.

Because God says he will separate the goats from the lambs (his children)

It was probably sacrificed for the devil?

Satan has a fabulous sense of branding