My guess then is that the gate agent did it, the agent probably saw you had a window and figured you were declining C+ upgrades just to avoid middle seats, and since there was a window available that you wouldn't mind.
Several gate agents I've talked to said they won't upgrade people at the gate into middle seats without asking. I am not sure if they can even see if you've opted out of C+ upgrades but there are people in this forum who could tell us.
This makes sense and in the case of this flight I didn't mind the seat change, but there are exit row seats that are superior to comfort plus seats.
If this is true it definitely makes sense, Middle seats should not be included in automatic or GA upgrades to begin with