What drives a girl to interrupt a strangers attempt to talk to a girl who to her is also a stranger?

This is a hypthetical scenario.

Friendly guy is flirting with a girl and working on getting her interested. She is just shy.

Two other girls they don't know had to interrupt and start yelling at him to leave her alone and starts threatening him. He leaves before the two rude girls behavior made the situation even worse.

Would you assume its racism and they ruined everything because he is black or do you think its something else?

What drives a girl to interrupt a strangers attempt to talk to a girl who to her is also a stranger?
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