Why does this girl become extra touchy feely when I’m around her and her boyfriend?

Like if he laughs at a joke I make for example, she starts to take his arm and start rubbing it in a kind of territorial way. There is a history - he has teased me, flirted etc in the past but nothing materialised. Even now he still sometimes steals glances at me or stares intensely when I’m not looking. Is this girl threatened?
Why does this girl become extra touchy feely when I’m around her and her boyfriend?
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