I understand that airlines block seats for... reasons. But why would half the plane consist of unavailable and unselectable seats? And why the pattern?

It's a long-haul flight that's consistently been flying the same metal, so I don't think it's a matter of protecting them from down-gauge.

This is for a flight in a couple days, so I don't think they're reserving seats to sell for more $ later.

My thought was maybe load will low and they're trying to do 3 seats for 1, but somehow people have already selected a window seat in the 'unavailable' pattern. Or weight distribution, but still allowing a family of 5 to select seats together?

Today it looks like this, but yesterday, they had several full rows blocked instead of vertical blocks:

https://imgur.com/a/gArtqEp