This isn't the first time says Rihanna. According to the police report, Rihanna told police that this wasn't the first time Brown hit her saying it was an ongoing and escalating abusive relationship.
The newest version of what went down (according to the "police report") goes like this… Brown got a text message from a woman who mentioned getting together and the argument started. Rihanna says Chris started hitting her and she ordered him to take her home and even "faked a call" saying things like "He's dropping me off. Make sure the cops are there." This call made Brown even more mad saying "You are really fu**ed up now. I'm going to kill you." That's when he started using his fists and Rihanna threw the keys out the window.
The criminal threats charge against Brown is based on the "Im going to kill you" statement and a source in law enforcement says "It's kinda weak, and the D.A. is not going to prosecute a high profile case like this unless it's solid." Note: This is based on a TMZ story.