Boyfriends brother put his arm around my shoulder why would he do that?

Me and him are neutral. We talk only if we hangout together but we not close either. So it caught me off guard when he did that. It was very random. Its been twice he said me what me and his brother (my boyfriend) did at night. Its not his business. I never think he would do that.
Boyfriends brother put his arm around my shoulder why would he do that?
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