Originally Posted by Smirnoff

To the OP, there are a variety of reasons why your seat may have changed (or have been changed).

B. Deliberate but Anonymous Seat Shifts
Another reason is someone changing the seat (maliciously, or for fun, or a BA insider doing it to put someone else in your seat). But to do this with no trace, would require them doing it in in MMB (so having your PNR and surname - very easily accessible), which should result in your receiving an email confirming changes to your booking. Always look out for these, if you haven't made any changes yourself.