1. Trying to provoke a response from higher level characters.
2. The ganker has, let us say, issues and/or is probably so bad at PvP that killing lowbies is all they can do. Often very young kids or intoxicated adults.
3. So big into Role playing that they believe it is their duty (really weird in my opinion).
Ganking and griefing isn't PVP in my books. IMO PVP is when I match my skill against someone else - the thrill is in wining against someone that could have conceivably killed you. Not much "skill" in jumping on someone that is so far below you in level that they pose no threat to you. Really quite... pathetic IMO.
Griefing is again IMO quite sad. "Tea bagging" or dancing and emoting on corpses, following people around, etc - again not mature and not what I would do. This is not an indication of PVPers as a whole but rather a reflection of that particular person and the way they chose to conduct themselves.
Unfortunately a lot of respectful people are turned off by this kind of behavior and refuse to PVP because they buy into the belief that this IS PVP - no it's not. It's someone choosing to demonstrate the "strength" of their character, their lack of manners and respect for their opponent, and how they were brought up.