I've often seen this when booking with an infant.

Last year I would only see alternate sides offered to the with-infant passenger: 2DF, 3AC, 4DF, etc.

I have a couple of upcoming booking this year in Y and CE, 320 and 319, and although I can see most of the cabin free, if I try to switch to the "wrong" side I get an error message and the change doesn't hold.

On one of last year's bookings, when the curtain moved the first row of Y (A&C only) fell on the wrong side, but I was able to call and get us moved there anyway. So whatever it is can be overrided.

I guess it is a policy (to distance infants a little) or safety requirement which has been coded in and is now either unknown or redundant to BA CS.