I would add ignorance and intolerance ( they often come together ) as a common reasons of bullying.
For example, others bullied me at school for:
- Mixed race (i.e. I didn't belong to locally dominated race);
- Weird emotions and weird ways to express them;
- Weird and clumsy social interactions (actually, I just didn't understand, what kind of behaviour others awaited from me in different social situations);
- Weird views and interests, which others didn't share;
- Clumsiness and weird walking;
and many other weirdnesses.
It is interesting, that I've changed several schools (but notice - they all were common level schools, with a lot of low-culture people and stupid people in them), and this situation re-appeared in new schools again and again. So parents and teachers concluded from that, that reason of everything lies inside of me, and this all is my fault. They all said that I should change, or I will be mocked and beaten (deservedly!) again and again through life, and nobody will be friends with such a freak of nature.
My mom said once, that when she look at how I walk on the street from the side, my walking look so ridiculous and pathetic, that she herself want to kick me. Also my mom and classmates were deeply sure that I'm crying and screaming from pain in very funny way, so, of course, they were happy to give me a reason to do it. And, of course, everybody were sure that I regularly spill liquids and break things not because of my clumsiness, but for a purpose to make others mad. And that I have weird interests not because I'm sincerely interested in them, but just because I want to stand out from others and show that I'm higher than they L(° O °L). The same goes from my weird social behaviour - everybody were sure, that I'm either too stupid, or like to play a clown and cause mocking on myself to draw attention.
Of course, I tried to change, but had no idea, how, because nobody was able to explain it in logically coherent way to me. So I always failed, and as a result, was sure that my life will remain miserable till I'll die... (I really wanted someone to just come and kill me and end all of this. Yes, since then I'm so passionate about killers
)
Since times, when I've attended the high-level University and meet a lot of civilized people with high intelligence from different regions and cultures, nobody bullied me anymore... not that I changed much, it more like these new people around were too clever for doing such a stupid things. It turned out, that smart people just don't think that if someone differs from them, they should torturing this person for that. Also there turned out, that a noticeable amount of other (very talented!) students and professors at my math department have their own weirdness, so they are more tolerable to weirdness of others. 
Of course, some of these people still didn't like me much, but the main difference was is that they didn't think that it is the reason to do bad things for me. They just didn't got friends with me, but instead got friends with people they like (it's logical, isn't it?) and left me be like I am, that's all.
In the same time, people, who liked me, just calmly ang logically explained me things about social interactions, which I didn't understand before, instead of yelling or mocking at me for that. So I've started to understand such a things much better and learned how to behave more socially adequately with time. Also people explained logically, how to grab things and move more correctly, so I wouldn't break everything so often. It appeared to be, that I just didn't notice some details about my movement, and when clever people explained me them, I've learned to move better, almost like the normal people do. Everything what I needed - just a reasonable, logical explanation.
So... eventually, I concluded that doing bad things to person, who differs from you, is often just a sign of low culture and ignorance in collective.