Bullying is usually caused by someone in power, wether physical power (strength, height, weight, or all), or power of a group (think cliques, jocks, mean girls) are casting it towards those with lack of power, a minority.
An example would be bullying someone for not having a brand of expensive shoes. The shoes being the thing they’re being made fun for,most likely due to socioeconomic status (power), towards someone who can’t afford it.
Or a traditional big bully (or bullies) picks on a small and weak kid.