Why is he acting awkward all of a sudden?

My manager’s brother has been acting weird around me. We recently had a work outing that he joined and him and I had a conversation at one point (2nd time ever speaking to him). It went well, it was casual, he’s great to talk to!

Now, I see him pop up at our office and when he tries to speak to me, he’s very shy and cannot maintain eye contact or a conversation like last time.

No, he was not drinking at the outing, so I know it wasn’t that. He was totally fine at the outing, and now he started acting differently afterwards, why?

Why is he acting awkward all of a sudden?
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