There is nothing that you can do to find out what really happened and it does not matter at this point as the room you wanted was apparently not available. Whether this was a tech glitch, someone at the property double-booked and would not admit it or somebody paid off the manager to dump you, is nothing the property will admit to and nothing Marriott can really do about it.

This then turns to the proper response. Is 10K enough? If it is, that is the end of it. If not, you should make a very specific proposal and focus solely on the end result, e.g., that you did not get your room, rather than what the property gave as a reason.