Here are some more interesting tidbits on the origins of passive-aggressive behaviors. Apparently, these traits originate in early childhood in response to not being able to openly express one's feelings around an overly-controlling parent.
Since the kid cannot directly express anger, he/she learns to express negative feelings in an indirect and subtle manner (for instance, purposeful 'forgetfulness.' Moreover, these maladaptive patterns continue into adulthood.
The roots of passive-aggressive behaviors are mostly ingrained by the 6th birthday. When a child grows up with an overly controlling parent and becomes overly dependent on that parent for decision-making and/or to meet his needs, the child dares not express anger toward the overly controlling parent for fear of rejection or reprisal. Instead, the child will almost certainly learn to express anger toward the parent in typical passive-aggressive ways without knowing he is expressing anger toward that parent.