I remember doubling 10th grade. As I entered the new community of new grade 10 students, I noticed that one girl was being bullied. I did not understand why. I asked the other students about it, but they were never able to explain it to me. Some didn't understand either, but had accepted the situation as normal. Others, all from the same village as this girl, seemed to participate more actively. Somehow, sometime during their elementary school years, the culture formed by the students of that year in that village had to come to include bullying that girl. There might have been an origin to it all, but when I entered their subculture in 10th grade all that was left of that narrative was an unexplainable shared dislike of that one girl.
1. Different (out of community, different religion etc.)
2. Not good with words (doesn't reply back)
I was in category 2 and a few bullies always made fun of me because of this. I was called names and humiliated for no reason.
IMHO, they don't need a reason. Just someone to make fun of.