This can happen when a ball or other object strikes the tip of your finger or thumb, and forcibly bends it. This condition is caused by an injury to the extensor tendon at the DIP joint. It can also be caused by a laceration, or cut, to the back of your finger. But most commonly results from direct trauma, often during sports when a ball hits the tip of your finger when it’s straight.
This can violently flex the tip your finger, rupturing the extensor tendon. In some cases, the tendon is pulled away from the bone, breaking a piece of bone from the tendon’s attachment point. This type of injury is called a bony mallet finger. Or as I stated previously, the tendon can break off without bone, leaving you with a tendinous mallet finger.