http://www.counselling-directory.org.uk/counsellor-articles/what-is-passive-aggressive-behaviour
Passive aggressive behaviour takes many forms but can generally be described as a non-verbal aggression that manifests in negative behavior. It is where you are angry with someone but do not or cannot tell them. Instead of communicating honestly when you feel upset, annoyed, irritated or disappointed you may instead bottle the feelings up, shut off verbally, give angry looks, make obvious changes in behaviour, be obstructive, sulky or put up a stone wall. It may also involve indirectly resisting requests from others by evading or creating confusion around the issue. Not going along with things. It can either be covert (concealed and hidden) or overt (blatant and obvious).
A passive aggressive might not always show that they are angry or resentful. They might appear in agreement, polite, friendly, down-to-earth, kind and well-meaning. However, underneath there may be manipulation going on - hence the term "Passive-Aggressive".