The answer to this question lies in the traumatic experience that Johnny had of being beaten up by some Socs, one of whom had three rings on his hand as he punched Johnny. As the blue mustang drives up to the group, Ponyboy notices that Johnny begins to breathe heavily and that he stares very intently at the Soc's hand in the car. Note what Ponyboy tells us next:
I remembered that it was a blue Mustang that had pulled up beside the vacant lot and that Johnny's face had been cut up by someone wearing rings...
Johnny therefore becomes so nervous precisely because he recognises the same people that beat him up so badly before the novel begins. This of course was a massively disturbing and traumatic experience for Johnny, and as a result, he acts in a very nervous way when he sees these individuals again now. Being placed face to face with your abuser is never an easy situation, and this is why Johnny begins to behave so strangely.
Those were Cherry’s boyfriend car,.a Socs. They met at the movies and new he is drunk.