Many Orioles fans already find Bryce Harper to be a player who should get booed on general principle, what with him being a player who projects the persona of kind of being a douchebag, and, much more significantly, being a good player on a team that is geographically close to the Orioles and possibly/probably/definitely is currently trying to screw the Orioles out of tens of millions of dollars of future television revenue.
There are also many Orioles fans who don't find Harper to be automatically worth booing, a stance that I find puzzling but can ultimately respect.