Happens to me all the time on EDI -> LHR. After booking they put me in a middle seat towards the back of the plane. NO idea why. I just select an exit seat, then T-72 (well,T-71:59:59) I normally get a row 1 seat.....

THere is probably logic to why they do it, but with OW Sapphire (BA Silver), 1 minute on MMB and you get the seat you want (for the most part).....