In general (and in a somewhat simplified translation from "legalese"), negligence means the failure to exercise reasonable care. When that failure ends up harming someone else, the negligent person can be held liable for the injured person's damages (medical bills, property damage, pain and suffering, and other impacts resulting from the accident).
Negligence is based on what lawyers call the "reasonable person" standard. That means, if a reasonable person would have taken a certain action in a certain situation, then not taking that action amounts to negligence. Alternatively, if a reasonable person would not have taken a certain action, then doing that thing would be negligent.
In order to determine whether someone acted negligently, a jury (or, in the context of most vehicle accident cases, an insurance company, at least initially) examines all the circumstances relating to that person's actions in connection with the underlying incident.
In the context of a motorcycle accident case, all drivers on the road have a basic duty to obey the traffic laws, be aware of what is around them, and respond in a reasonable manner to those surroundings. That includes being vigilant and on the constant lookout for obstacles and other variables on the road -- other vehicles, pedestrians, traffic control devices, debris, bad weather conditions, and the like.
Let's take a couple of examples of "no contact" motorcycle accidents to see who might be found negligent.
A car and a motorcycle are both traveling in the same direction down a two-lane street, each vehicle in a separate lane. The car is a little ahead of the motorcycle and switches lanes quickly, without using turn signals or otherwise giving any notice to the motorcyclist. In order to avoid the car, the motorcyclist swerves, loses control of the bike, and crashes, or just lays the bike down. There was no contact between the car and the motorcycle. In this situation, it is likely that the driver will be found negligent. The driver switched lanes without signaling an intention to do so, and prior to that, the driver could have easily seen the motorcycle by making a reasonable effort to determine whether a lane change was safe to execute.
Now, let's say the motorcycle is traveling behind the car in the same lane. The car slows down, but the motorcycle does not notice the deceleration, and maintains his speed. At the last moment, finally noticing that the car has come to a near-stop, the motorcyclist swerves and/or lays the bike down to avoid a collision. Who is responsible here? This accident is like a rear-end car accident, except that there was no actual collision.
In rear-end collisions, the tailing vehicle is almost always going to be liable for the crash. This example is no different. In this case, it is the motorcyclist who had the obligation to see what was there to be seen and react accordingly. The accident happened because the motorcyclist failed to notice the car slowing down and had to lay down the bike in order to avoid crashing into the car. In this example, there would likely be no valid personal injury claim against the driver. The motorcyclist was probably negligent and would likely be deemed solely responsible for the accident.
Looking at these examples, you can see that "no contact" motorcycle accidents are just like any other kind of traffic accident. The issue is not whether the two vehicles collided, but whether the drivers or riders acted negligently. Bottom line: If the negligence of a driver causes a motorcyclist to crash his/her motorcycle, that driver will be held liable to the motorcyclist, even if the car and the motorcycle do not actually collide. Learn more about common causes of motorcycle accidents.