Based on my recent experience, the unbooked preferred seats (usually middle seats) are allocated to passengers who did not prebook seats, did not do OLCI and showed up at the airport to check in. On my last 3 A320 flights, it always involved a family with infants or babies, who would start wanting people to move so they could sit together. While most people will not move, on 2 occasions (somewhat in self defense), someone ended up moving from an aisle to a middle seat after the fact.
It is my opinion that AC has created a problem here that it needs to solve. It might be tough to tell such passengers that the only seats left need a fee to be paid - but not much different to telling someone that their carry-on is oversize or weight and needs to be checked for a fee.