An observation as a BAEC Silver member:

Just went in at T-72 to select seats for myself and Mrs Makk1 (BAEC Blue, separate booking) on the LGW-MLA flight. CE 1A+C showing as free so juggling one booking on the ipad and the other on the laptop I go to seat selection. Move myself to 1A but when I go to the other booking 1C is now not showing as free (but still is on my booking). I have to move myself out of 1A then move Mrs Makk1 into 1C before moving my seat back to 1A.

So clearly some attempt to keep a free seat next to me as a lowly silver! May also explain why, somewhat to my surprise, the WT+ bulkhead middle seat next to me on my 787-9 TATL flight last week remained free.