Maybe they're blocking them for tour groups? Those groups usually sit together in the back, and I believe they get their seats assigned at checkin.

They look "blocked" on expert flier until two days before flight. Then they fill up. Way back when I was flying my first CX LAX-HKG flight in Y I asked this same question (my first FT post) hoping to get four across in order to stretch out. My hopes were dashed as the "empty" seats miraculously filled the last couple of days. However I was lucky in that the nice counter agent blocked the two seats next to me giving me three seats. I got a lot of dirty looks (almost full flight), but was quite comfortable.

Yes, they are blocked until OLCI which opens up 48 hours before flight time.