I first noticed this years ago while travelling in the US, and while there was still assigned seating. The only thing I could think of was it was an attempt to secure a place for the vast array of hand luggage that was going onboard. I don't know if it's still that way over there, but perhaps the idea spread beyond US borders.
Remember flying from Washington DC to Chicago about 14-15 years ago and they started announcing whom should come forward, FF Platinum plus, FF Diamond, FF Gold, FF Silver, FF from Partner airlines etc etc..............they all boarded by whatever FF class they had.
Ultimately it was just myself and a young lady left after all FFliers had boarded
We were called forward and the two of us looked around and asked you sure you got all your FFliers.........fit of giggling hit the 2 desk people.
They said its always this way as taxpayers pay their fares, you have to read out every single one as someone will get upset, this being Washington.
They tickets were taken off both of us and they said well neither of you are FFliers with us but seems we have some upgrades, introduce yourselves as you both just got upgraded.
Of course when boarded there was no space in overheads as people down the plane seem to always want to leave their bags as close to front as possible..........seems a US thing though.