There might be a reason, but it's probably not a good one. These things are chaos-theory-like[1], with the causes so tiny that they're impossible to observe or predict and so may as well be random. It could be as simple as "One day, someone was in a bad mood and took it out on the nearest person unlikely to fight back. They realised that they enjoyed it, and did it again. Others observed and joined in." - by the time it reaches that point, you're already talking about group bullying by multiple people. Knowing the cause, or that there even was a cause, doesn't really help anyone.

[1] http://en.wikipedia.org/wiki/Butterfly_effect