Why would a usually friendly guy be suddenly slightly rude and angry towards a girl?

When I saw him in the coffee kitchen yesterday and greeted him he seemed surprised but not really pleased to see me which is very unusual for him since he is very polite and seemed to like me. I ran into him today again and didn’t greet him at first because I didn’t recognize him from afar. When I said hello he replied in a very angry tone and left quickly. I never heard him talk like that to anyone. What is his problem?

Why would a usually friendly guy be suddenly slightly rude and angry towards a girl?
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