I am flying Air NZ business on the 777.

Selected 7K as I've had it in the past and I like it. The day before departure I had a look at the app and saw that my sear had mysteriously changed to 9K. Tried to change it back to 7K as it was showing as available but every time I did so the system just popped me back into 9K.

Fast forward to today and when I went to check in I asked the check in agent if 7K was available. Yes was the answer and I was given a boarding pass for 7K. At boarding they are paging my name and when I get to the counter they tell me "sorry we have to move you to a different seat". "Ah, I see" mesays, "the seat must be broken". "No", I am told. "Someone else is sitting there". Sure enough when boarding, I see that there is someone in 7K.

Out of curiosity, learned folks, what do you think caused this? I am a bit miffed to be fair. Is it because I am lowly Gold and an Elite member asked for my seat?

it was probably inop and then moved back to operational. Alternatively it may have had say a broken tray table and they offered it to a pending upgrade pax with this information.

I had even more confusing one back in May - I got all the way on to the aircraft (777 heading to Houston) with both a printed and digital boarding pass showing I was in 1A, only to find another passenger already sitting in 1A - and she also had a printed boarding pass for that seat.
Crew didn't seemed surprised, and commented that it happens 'often' on that aircraft - luckily they were able to find me another business class seat.

If it were an aircraft swap, 9K is your new 7K, so no need to be too alarmed..

disregard (deleted)

same has happened to me, twice, travelling on full-fare BP. Planes not even full, but told I couldn't take my original seat (4A) because another pax wanted to be able to sit next to a companion. Pretty annoying to not even be asked if I was OK with the seat swap. NZ just did it.