The Boomerang effect describes a social psychology situation in which a person who is presented with a persuasive message and then adopts the opposite stance as a result. They essentially "boomerang" to the other side which is where the name of this phenomenon comes from.
Research suggests this occurs because when a person feels their freedom of choice is being infringed upon so they sometimes react by going to the opposite stance. An example can be seen in public messages. If a message is too strong or forceful then it is not as effective. For example, if an anti-litter sign is too forceful and says "You must dispose of trash" people are more likely to litter than if they saw a more discreet message that says "Please dispose of trash."