Why does Jem suddenly reveal all that has happened to them?

Jem reveals all that has happened to them in the hope of protecting Scout after Bob Ewell threatens to harm her. He wants to convey the truth about their attack to Atticus, fearing that Bob Ewell may try to carry out his threats, and to ensure that they are safe from any further harm. Jem also wants to seek justice for their ordeal and hold the real culprit accountable.