In chapter 10 why did Ponyboy end up in the hospital?

Ponyboy ends up in the hospital in Chapter 10 of *The Outsiders* because he is stabbed by Bob Sheldon's older brother, Randy.

This happens during the rumble, when Ponyboy is separated from his gang. He encounters Randy, who initially shows some remorse about the events that led to Johnny's death. However, the fight erupts and Randy, overwhelmed with grief and anger, stabs Ponyboy.