I find it interesting that the conductor did not lock the switch. I don't know American regulations but the Canadian Rail Operating Rules are quite specific. For hand-operated switches: Except while being turned, each switch must be secured with an approved device. When a switch has been turned, the points must be examined and the target, reflector or light, if any, observed to ensure that the switch is properly lined for the route to be used. - Rule 104(b).
Often the switch is secured by the hasp of the lock. The problem with leaving a switch unsecured is that the movement of cars through the switch can cause the switch points to move. Putting something in the switch mechanism like the lock hasp prevents the switch from changing position.
Oil-Electric goes into the wreck and location in more detail... check it out!
EDIT: There is a news article about this derailment.
EDIT: I initially said it was a trailing point move, but the overhead shot from the above news article makes it clear it was a facing point move, i.e. the train was going INTO the siding. An unsecured switch could easily move in this case.