passengers are not supposed to be able to block off bulkhead and or exits seats themselves. because of safety and security restrictions they are reserved for the gate attendants to dole out.

If you request bulkheads or exits, they will note it for you BUT it does NOT guarantee that the day and time of your flight that the airlines won't NEED to relocate you. They will always relocate you from bulkheads if they need the space for families with small children, or handicapped. Families with small children are prohibited from exits as is anyone with physical or mental challenges due to the nature and purpose of WHY there are Exits.
Lots of times SOME of these Exit Seats as well as other SPECIFIC seats will be requested by U.S. Air Marshalls and once a Marshall is aboard They
(on official business) outrank even the Captain.

Your exit may well have been requested to be held for the Marshals.
There can be one aboard or half a dozen aboard a specific flight.