Why do guys get awkward all of sudden?

He is a very confident guy and we talked few times already he was always nice, but suddenly he got so awkward around me and it was even difficult for him to say *hey* to me - he kept putting his efforts into ignoring me. And send his friend to talk to me instead, then he came over and was really arrogant but at the same time awkward I reacted by getting bitchy so he left and haven't talked to me anymore.
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Thanks for the answers, the thing is also when I was leaving I saw him talking to some random girl
Why do guys get awkward all of sudden?
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