I think the problem with this thread is in bracketing bullying and being harassed together. They are not the same thing.
The majority of children will be harassed at some point in the their time of school. The majority of children will also harass another child.
Actual bullying is much rarer and it's usually easy to see why it's happening. I don't believe that it's very often about luck (clearly there are exceptions, some of them on this very thread).
I think partly it comes down to a child's attitude and perception. Children (even older ones) are not always able to accurately describe what they are experiencing. Many use the term bullying far too lightly. For example, I have a pastoral tutor group which includes two children with SN. I get frequent complaints from other children (and sometimes even parents!) that these children are 'bullying' them. They absolutely aren't. They've simply misinterpreted a social situation, lost control of themselves or got themselves stressed out. Their behaviour is never targeted against an individual for being that individual. That to me is the difference between harassment (regrettable, needs sorting but happens) and bullying (needs stamping out).
Reasons why children get bullied (ime):
*they are visibly different in some way that goes beyond mild teasing (glasses, braces, extreme height, extreme weight, red hair etc)
- they are socially different in some way (quirky, SN, overly confident, under confident, abrasive etc)
*they have an unusual home background/culture
*they are very clever or very low ability (depends on the school which is more likely to seen as negative)
*they have unusual extra curricular interests or interests that are seen as unacceptable to or different from the main stream/dominant school culture.
*they have no self esteem or armour to protect themselves and allow harassment to become bullying due to their reaction.
I think that, if a child manages to escape all the above circumstances, it's unlikely that they'll be systematically and consistently bullied. Of course many children in the above categories won't be bullied either.