I'm sorry my answer wasn't what you were looking for but it is by far & away the most likely reason. BA don't kick people out of exit seats to accommodate higher status passengers. Prince Harry & Meghan Markle were dumped at the back of economy recently on a flight to Nice so I don't think a random GCH has had you moved.

At some point a change in plane or predicted CE load has occurred and you were moved. As I mentioned, the system is incapable of shifting you back once the exit row became available again. I'm not defending BA for this, it's incredibly frustrating and has happened to me before but on a short sector I try & not let it bother me assuming I don't end up in 19D but theoretical seating should protect you from that as a Silver.

The only way to guarantee your seat is to check religiously at least daily in the fortnight prior to the flight when most changes and seat reservations occur.