Both times the other person said something like, "Oh I am in seat so-and-so" and moved, without actually checking their boarding pass, to another MCE seat. Seriously?
All that tells me is they were not in doubt about what seat number was on their BP. So either they misread the seat number placards on the plane (so now they re-check
those and move),
or they intentionally tried to poach a seat. (It's conceivable that they also lied to you about their actual seat and just picked another still-empty seat to move to and stay in MCE, but you don't mention them being subsequently confronted by anyone else.)
Both times the person was a solo traveler, so nothing like a "please switch with me" scenario.
Why does solo preclude a desire to switch? They might want left side vs. right, or window vs. aisle, and not have been able to get their preference. That doesn't excuse preemptively taking someone else's seat, of course, but it could be a motive.