rebooked on an alternative carrier
This is not the same as sub contracting a flight. Any loss or damage to luggage is the responsibility of the carrier who's flight you are on - the carrier issuing the ticket is generally irrelevant to such claims, I think you will find (and which Mr Fogg HAS found!).
Back to the thread "Operational Reasons" is a useful catch all phrase that could mean just about anything from "We've run out of serviceable planes", through "the pilot got held up in traffic", "It was foggy yesterday morning and we haven't caught up yet" to "We're only the handling agent and we don't have a clue why but it's going to be late/cancelled."