Theres a lot of rumours flying around.
Chris got a booty call, so she slapped him numerous times, and he then assaulted her.
Chris assaulted her because she gave him herpes. (I'm not making it up, its really a rumour)
It's not even 100% confirmed it was Rihanna, as of yet. Hence "allegedly" in the title.
Bottom line is, as a man you can restrain a woman, but you do not raise your hand to them in an aggressive manner.