The common, perhaps universal, pattern of bullying is that "the different" are targeted. Three possible evolutionary mechanisms, attempting to answer a previously asked question.
1. Bullying tests whether someone is in the tribe, to discover whether they can be trusted. How would one pass the test? This could be taught.
2. Someone has already been identified as not part of the tribe, someone who has allegiances elsewhere so cannot be trusted. Bullying serves to expel them from activities and benefits of this tribe.
3. Bullying may serve as incentive to abandon their old tribe and maintain allegiance to the new one.
Attempts to regulate bullying seem doomed to fail, as they seem to be trying to regulate human nature.
People are different in many ways. Which differences are selected for bullying, and which are not? Why? Assuming correct #2 above, it would be the differences, especially behavior, that signify tribe membership, whether someone can be trusted. These behaviors might be subtle, and may only happen to correlate with the popularly believed reasons for bullying (e.g., race, gender, body shape, sexual orientation).
If someone is selected for bullying, how accurate are the bullies' determination that they are not part of the tribe, someone who cannot be trusted because they have allegiances elsewhere? I suspect it is frighteningly accurate, but if not, this could be taught to be more accurate, though such teaching is politically incorrect.