Two of my girlfriends and I pinned my older sister's boyfriend while we were playfighting. She started it. Why is she mad at us?

She has her boyfriend from college over, encouraged us teasing back and forth, and even got him to push us (in a joking way) and was basically getting us to pick on each other back and forth. When it got physical and we started wrestling around, she thought it was funny. But once we got him down, and kept him down for about 5 minutes, she suddenly changed her attitude and now she's really pissed. wtf...
Two of my girlfriends and I pinned my older sister's boyfriend while we were playfighting. She started it. Why is she mad at us?
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