Layouts and Styles can also apply the format lock, if they are configured that way.
But a script does seem more likely, as I think there were discussions about applying the format lock via a script recently. Maybe Search results format lock
It's also possible it's both: a script may have turned it on and you then saved the default lister / layout / style (to save something else, without realising it was locked) so it's locked again when they're loaded.