I recently flew economy class with British Airways, short haul within Europe if that makes a difference. My booking gave me the ability to either pay to reserve a specific seat or wait until 48 hours before departure when I would be able to reserve a seat for free. Note that other ticket types only allow you to reserve your seat for free 24 hours before departure, so my ticket gave me a small advantage over these other passengers.
Looking at the available seat choices when I booked a few weeks before flying (and at some intermediate times as well) it certainly seems like the paid-for seat choices are not that popular with most seats still being available ~60 hours before departure. However, once I was within my 48-hour window, I was only offered a choice of just four seats. I was left wondering how all those unreserved seats got reserved so quickly, and what choices would remain if my ticket has restricted me to the 24 hour window instead, or I had waiting until I was at the check-in desk. Strangely, pretty much the same thing has happened on other recent flights with the same airline, with a far smaller choice of seats that I might have expected to be available.
What occurs to me is either a hundred or more passengers are eagerly waiting for that 48-hour window to open to 'get in' and reserve their favourite seat, so most the seats are being reserved for free within a few minutes; or British Airways are not actually offering all the currently available seats to those passengers eligible to reserve a seat at that time.
Is information available somewhere that gives details about the online seat reservation strategy adopted by British Airways?