It will depend on state laws usually to determine who would be at fault if a driver slams on their brakes causing the car behind them to hit their vehicle. Most states have negligence laws of some type and with these one or both parties might be found at fault.

In some states they could determine driver B to be at fault since the driver of Car B should have been leaving stopping distance so that if Car A slammed on their brakes Car B would have room to stop without hitting Car A. In other instances if it can be shown that Car A braked on purpose to cause an accident that driver might be found at fault.

Or finally both Car A for purposely slamming on their brakes and Car B for following too closely, tailgating, could both be found at fault. You will need to research the negligence laws of your particular state. For example if it has comparative negligence laws than that is when two parties fail to perform ordinary reasonable care, which if this was a road rage incident could likely be shown.

Your insurance agent with whom you or the other driver might be putting a claim through should be able to help you with this. If claims are put through both driver's insurance companies, both providers will likely investigate the incident and determine fault.