Why is this guy being kind of "hostile" to my friend?

Context : me and guy A had a thing before romantically but he decided to leave me and kinda ignore me because he said that he was not ready for serious relationship. This was last year.

Now me and guy B are friends. But some people thought that we are together which are not the case. We are just friends.

But this month recently everytime guy A is drunk, he saw guy B , grumbled, mumbled, and told him that "I don't care if you are together with xx (my name)!" . He said this everytime he saw guy B when he himself is drunk. Guy B told me this , as he was making a point that we shouldn't be seen together again as some people interpreted us as being together. He also told me to tell guy A the truth.

Three of us are classmates, and , I notice that guy A is kinda nervous around me. He is fine with other people but kind of nervous around me. One day guy A was drunk and he was just staring at me for long time.

What happened with guy A?

Why is this guy being kind of "hostile" to my friend?
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