What causes this?

poorly adjusted shifter - the neutral safety switch keeps the car from starting, or trying to start and lurching forward or back. It is in the transmission and if you are not completely in the right gear (park or neutral) it won't allow the car to start. If your shifter isn't adjusted properly the shifter may show it in the right place, but it is a bit off at the transmission. All shifters have an adjustment to get it right. usually best to disconnect the shifter at the arm on the trans, put the trans solidly in Park, put the shifter itself in the right spot and then adjust the linkage so the pivot at the end of the shifter linkage passes smoothly and easily in the arm on the trans without pushing the arm out of position. The detentes in the trans are very positive and cannot be misread as you go from gear to gear by hand. It is way better to do this with two people - one in the car and one under the car