Honestly, most accusations of passive aggression seem to be from people used to more blunt social mores towards people with more circumspect ones.
But passive aggression IS real, and comes down to one thing: Trying to have conflict without confrontation; trying to attack or criticise others without allowing for response.
Phrasing a request by stating a desire that someone do the thing is not passive-agressive. Writing an angry anonymous note IS passive-aggressive. Criticizing a problem to someone who can fix it is not passive-agressive. Criticizing someone for a problem to a third person IS passive-aggressive.