In court, the person who gets sued or accused is called a defendant — they have to defend their innocence or reputation.
One thing no one wants to be is a defendant: that means someone sued you, which could cost you a boatload of money. Other defendants are accused of crimes, which is even worse, because you could end up in jail. Turn on any TV show about lawyers, and you'll see some of them defending the defendant, and others trying to convict the defendant. Defendants are definitely on the defensive.