A system called "Theoretical Seating" kicks in 72 hours (and 40 minutes, to be precise) before the scheduled departure time of all BA flights. This allocates seats to passengers who are in groups and especially those with children, to ensure they can sit together. This system will have allocated you seats prior to check-in opening.
As an aside, it also blocks the seat adjacent to a BA Silver or Gold member to those with lower or no status until the flight becomes full - with Silver blocks disappearing before Gold blocks - to maximise their chances of having an empty seat. This is one of the most valuable benefits of BA status to me, even though by its nature it is unpredictable whether you get to benefit from it on any given flight.