My daughter broke her collarbone when she fell off her bike. She was 10. What I don’t understand is why her collarbone broke because she put her hand out to stop the fall.

Collarbones important bones that help support the arms. They can be broken either by a direct hit to the bone or through an indirect one. In your daughter’s case, it sounds like an indirect break. When she put out her hand to stop the fall, the force of the contact between her hand and the ground had an impact on the bones in her arm and went up towards the collarbone.