A bumper humper is a man on man action when he would rather place his penis in other mens asses rather then have sex with a women. This could also be used as a phrase to simply call a man gay.
See that man over there wearing a pink shirt and kissing that other man buddy? He must be a bumper humper.
One who drives closely on another's bumper to intimidate such that the closely followed/bumper-bullied will change lanes (from fast lane to slow) to enable the subsequent bumper-bullying of those in front of him/her who might be merely driving the speed limit.
Bob was enjoying his coffee on his commute to Denver from Fort Collins, and though he was driving the maximum speed limit his rear-view and side mirrors served him a constant reminder of a SUV bumper-bullying him to change lanes.