Networked Mac keeps remounting itself

Just updated a 2014 MacBook Air to Catalina. Any idea why my iMac on the same home network keeps remounting itself in the MacBook’s Finder? Happens not just on reboot but seemingly randomly during the same session, even after I’ve ejected it.

Didn’t happen under Mojave.

Any suggestions for keeping the iMac from mounting until/unless I choose to mount it?