Imagine this: You are driving with a couple of passengers in your car and decide to use the high-occupancy-vehicle lane, since traffic is stop-and-go in the other five lanes. You move to the left and cross the broken white line in the area where it is legal to move into the HOV lane. Once you are within the restricted lane, the dividing line changes to double solid lines, indicating cars must not move into or out of the HOV lane until they reach the next broken-line area.
You and other drivers properly in the HOV lane are able to move considerably faster than the one-person-per-car vehicles in lanes to your right. You don't have to worry about other drivers making lane changes in front of you or so you think.
Suddenly, frustrated and impatient, a driver decides to illegally move into the HOV lane, crossing the solid line. That car was nearly stopped, and as it switches lanes, it is moving maybe 10 mph, while you and others in the HOV lane are going 65 mph or faster.