Any reason for Kavanagh and Marriner swapping.
Maybe they wanted a more experienced VAR on the Man City game?
Well, I don't see why they'd bother to make a change just for that. It's not like Everton V Man City especially warrants a specific VAR and whilst Marriner is more experienced as a referee than Kavanagh they probably have similar experience as VARs.
The most likely reason would seem to be that Marriner's not in a position to referee today for whatever reason (injury?) but can be a VAR.