I had this recently from LCY. Had the first row behind CE so I knew there was a risk of the curtain being moved and was fine with that. I had checked in, received boarding pass already so thought I might get lucky this time.

Was told at the airport that I had been moved in order to balance out the aircraft from a weight perspective.

Of course when I got on board the CE section had expanded and there was a (much larger) gentleman sitting in my previous seat.

Like I said, no problem with CE expanding (I know the risk), just wish they would have been honest at LCY and told me the real reason.