The same way another 2000+ pound car can cause someone to have an "accident". There are variations on the idea and of course other possibilities but picture if you will a cyclist swerving into the lane either from the shoulder or from an adjacent lane causing the traffic in the target lane to take evasive maneuvers which may result in loss of control of one or more vehicles and collisions. A person stepping out into the middle of traffic can also cause the same thing as could an animal.