It is a common myth among many that no matter what happens, no matter what the situation is, the vehicle which rear ends another is always at fault for the accident, and therefore liable for the damagers. While this may be an appealing thought to many, it is also a very false impression. There are many factors which go into deciding which vehicle, if any, were to blame for the accident.
There are a variety of circumstances that determine whether or not it was the fault of the driver who did the rear ending. While in general, it is quite true that most times it is the fault of the driver that did the rear ending, there are some circumstances where this is not the case. For example, if a vehicle makes an unsafe lane change in front of your vehicle, then proceeds to immediately brake afterwards, this may cause you to rear end the vehicle. However, you will often not be found liable for this, as it was the fault of the vehicle making the unsafe lane change. Another instance in which you may not be held liable for rear ending a vehicle is if your vehicle is stopped for traffic and is rear ended, causing you to collide with another in front of you. There are other instances where may not be held liable.
If your vehicle has one or both of your break and/or tail lights are out, then you will likely be held to be at fault for the incident, particularly if the incident happens during the night. If your car was having mechanical trouble, yet you did not do all that you could in order to move it off the road, you will once again likely be held responsible for the accident.
Often, regardless of whose fault the accident is, it is difficult to prove that the blame was on one driver or the other. This leads to many insurance companies taking the stance that the vehicle that did the rear ending was the vehicle at fault. This helps to simplify claims in the eyes of the insurance companies, making their businesses run just a bit smoother. While not the law, the vehicle that rear ends another is often the vehicle that ends up paying for the damages, unless it can be proven to be the fault of the other driver.