I've seen this too. Sometimes after force-quitting pgadmin a process associated with NW.js runs away using 100% of a thread.
It seems to be associated with first shutting down the underlying database server before quitting pyAdmin, but I haven't made an exhaustive investigation.
Sometimes when I "quit" PgAdmin on Mac OS Big Sur (11.3.1), the GUI goes away but PgAdmin continues to run. Even when I ask it to quit it still runs and I have to use Force Quit to stop it.