I think there are some reports on here that suggests the system itself suddenly kicks some or all people out of their seats. Could be something in operations that triggers an unnecessary re-seating (a real or imaginary equipment change) or the Altea departure control "optimal seating" having a bad day or being poorly configured.
There is also the case where the system kicks people out if it believes they do not have status enough to select seats for free - this one has happened to me where my reservation listed my AY number but claimed I had no status and all seat selections where gone. AY does kick people out of seats to accommodate their own (had that one too).

I have never heard of elites being able to kick other people out of seats already being selected though.