I think the likely scenario is that the car behind the witness car didn't have enough room to stop when the witness car abruptly pulled off to the right. And to avoid rear ending the witness car she swerved left.
That is what I think is the case. It's easy for people to say what they'd do in that situation, but she probably did go left to avoid the witness and unfortunately hit the victim. That is the scenario that's most likely. And, despite people saying they would never have swerved left, I call bull-shyt because you don't know what you'd do in that situation, especially when there's a speeding car coming at you head on.