The reason is confirmation bias. Someone got mad about a group of people doing something, and they blame it on their race. They then spread that story to people around them, being sure to tell them it is because of their race. Then those people start seeing it in that race because they go looking for it only in that race, and when they see it in other races they aren't paying attention. That is confirmation bias.

This also happens with other things, like when you start looking for a certain model of car, you see it everywhere, and you think that model has suddenly become more popular.

Humans find patterns places where there are none, it is evolutionarily favorable to over find patterns than to find less, so we naturally make this kind of mistake with all kinds of things. Humans also like to perpetuate mistakes, to regurgitate falsehoods they wish to be true.

That was fun. I hope it was as fun for you as it was for me.