When something is unwarranted it's not called-for under the given circumstances. For example, debating the merits of someone's talent is one thing, but calling them stupid is unwarranted.
I thought the applause was unwarranted. After all, all the guy did was stand there and point for two hours. And when that lady started screaming? That was unwarranted, too: all I did was give her a flat. It was an accident! Think of it this way: when the police want to search your home, they have to get a "warrant" — a document that allows them to enter. To go into your home without a warrant would be unwarranted. A warrant is a kind of justification; without that justification, something could be called unwarranted.