There are two reasons. First, he doesn't like Norton. Norton is always making fun of people. Lorraine is John's best friend, so it angers John and he gets defensive. He calls Norton names is return to give him a taste of his own medicine and make him think twice about disrespecting her again.
To go even deeper though, John seems like he has even deeper feelings for Lorraine beyond friendship. Their friendship is a curious one to begin with since they are such opposites, but when they spend time at the Pigman's house, the reader can see a romantic relationship being formed. Of course, the kiss is the obvious example of this, but the way they interact afterwards, John's bossing her around and their bickering back and forth like husband and wife, shows the reader he cares for her on another level which is why he felt he had to stand up for her against Norton.