The picture with the bolt was dismissed as possibly being an anomaly of the camera lens, but the other theory was that the shuttle someohow passed through an electrical field that caused particles oppositely charged from the plasma around the shuttle to discharge.
A bolt of this type could explain the sudden loss of numerous electrical sensors and the subsequent breach of the hull.
Or it could have blown the emergency explosive release on the left side landing gear, causing a chain reaction of events that eventually brought Columbia down.