I can give one reason for seat change ! Checking in for another code-share airline when I still qualified for B.A. Staff Travel concessions, I was give seat 96Z - or similar, right down the back anyway. I expressed surprise and remarked that I used to fly these things and didn't realise that seats went so far back ! Oh ! Really ? who did you fly for, said the ( relatively ) young lady, and than Oh, I used to fly for B.A. too, do you remember so and so, blah blah blah, and we chatted for awhile about mutual friends and experiences.
Then she asked for my boarding card and replaced it with a seat much nearer the front. Just before boarding an announcement was made for a Mr. XX and a passenger presented himself to the desk, and was advised that his seat had been changed, whereupon he was given a new boarding card for my original 96Z.
I kept very quiet and enjoyed my flight !