Yes this is normally the Marriott dynamic pricing engine at work or an error when they are doing maintenance. In the case of the dynamic pricing engine as room inventory drops the prices go up for the remaining rooms.
So if there is a convention or business group meeting and there a limited number of available rooms even if they are the lower tier rooms the rates may be nuts like your experience where saw rates above $1100 per night.
A group may have a large block of rooms on hold or
it could just be one night that has limited availability. Are you looking at the rate per night or the average price for the entire week?
You can just keep checking and the group may still cancel or release any rooms that their group did not book by their booking window.
You can see some examples on this page but the week of July 12th does not show as sold out:
https://swandolphin.com/value-days/
Note: The Swan & Dolphin are Convention resorts and they only offer the big discounts to fill blocks of rooms that would otherwise go empty when they have high availability.
Dave