You asked

How do bullies choose their victims?

The sad fact is that any child can be bullied just by being in the wrong place at the wrong time. Very often the thing that makes him included and accepted in one environment can cause him to be bullied in another environment.  Children who are popular, intelligent or attractive can be bullied but equally children who appear easy to hurt can also be targets of bullying.
Bullies generally target:
Anyone who appears different:be it their looks, weight, height, accent, the clothes they wear, the hobbies they have. Disabilities make some children an easy target.
Children who are small or young and unable to defend themselves.
Children that they can get a reaction out of: popular targets are children who are upset or cry easily.
Kids who are not sporty or who perform badly at school.
Anyone who is socially awkward or anxious or who struggles with shyness .

More questions

Children who bully want to exert control over their lives-even if it's just making someone else's life miserable.
This can be a difficult realisation to come to but there are some signs to watch out for which could indicate that your child is a bully. 
Your child's school should have a very clear policy on how to deal with bullying.
In recent times, cyber bullying has become all too common. 
There are lots of things that you as a parent can do to help prevent and reduce the chances of your child being bullied.
Bullies choose their victims for many reasons and the sad fact is your child can be bullied simply by being in the wrong place at the wrong time.