Sounds like you were moved to accommodate a disabled passenger as they tend to be assigned bulkhead seats (can vary by aircraft, but in general bulkheads are prone to this). Or could’ve been an ultra-high status pax traveling with a companion that an agent moved to sit next to in the “B” seat. (Not policy or a published benefit, but does happen from time to time). At any rate seat assignments are subject to change, but I do agree it’s frustrating when it does happen.

Day of seat swaps can also be due to deadheading crew contractual seat requirements or inoperative seats.

Lastly, good ole Delta IT is always a possibility as well.