The switch was not reset after the freight train used it to get off the mainline, and a derail was not set on the siding.
It was a CSX freight that last used the switch, CSX trackage & dispatch.
I submit a rewording of this point. The freight train was parked before the siding was padlocked unless it was padlocked before the train was moved, which is probably a stretch. I assume it would not be padlocked in the spur setting. That would then require a positive return to the main THEN padlocked. Looks like someone just locked the switch after the move without setting it to the main. May not be a whole lot different than what you said, but it
was presumably set, incorrectly, and that could be a meaningful difference.