Mechanical Cause
When an adequate centering force is not provided by the vehicle's detent spring, the detent system does not always force the transmission into a gear. On certain Chrysler vehicles, this problem is magnified by a "flat spot" on the vehicle's inner manual lever
Despite the introduction of "detent systems" and the "Key Shift Interlock" safety measure to help prevent roll-away accidents, there continues to be a large number of serious injuries and deaths which result from inadvertent movement.
These accidents fall into two major categories, each with their own causes, but both of which are resolved via non-override door locks or the use of an "out-of-park" alarm system.
INADVERTENT MOVEMENT
Vehicles can be mis-shifted into a gear or left in a gear where the vehicle is temporarily held in place by terrain
DRIVER MIS-SHIFTS
At some point nearly every driver makes a mis-shift and ends up in a gear position they did not intend. A Chrysler funded and directed study in fact showed that mis-shifts are extremely common. In a study of over 900 shifts, a considerable number of people either failed to shift or shifted into the wrong gear.
In normal circumstances after mis-shifting, when the driver then removes he/her foot from the brake, the vehicle does something unexpected, moving when it is believed to be in park, or moving in the wrong direction, or the car revving as the vehicle was inadvertently put into a lower gear. The error is quickly detected and the vehicle placed into the intended gear. However, very rarely, vehicles can be mis-shifted into a gear or left in a gear where the vehicle is temporarily held in place by terrain (usually a sandy surface or ground that will slowly compress). On paved surfaces, or hard flat dirt, these events are equally dangerous, if far less common. Mis-shift accidents are less common on paved ground as absent some unusual terrain (such as a large pot-hole) which could hold up the vehicle in gear, the vehicle will always move immediately upon a driver removing their foot from the brake after a mis-shift.
False park (hydraulic neutral)
The "Flat Spot", where shift into a rest position between park and reverse
Transmission "pop" from stasis into reverse, causing vehicle to back up after a few seconds
PARK TO REVERSE EVENTS
The second type of events are called "park-to-reverse" accidents, and occur on vehicles where it is possible to place the vehicle's transmission between gears. The vehicle then appears to be in "park" as it does not move, and then after a delay, the vehicle will reengage either drive or reverse and begin to move. When the driver has exited (to open a gate, get the mail, help a kid get into the car, etc) the vehicle can move driverless striking the driver or bystanders.
Poorly Designed Detent Systems
With the very first automatic transmissions, it was possible to place the shift selector at any point, either in an intended gear or between a gear. Because of the possible safety issue of this, and because driving a vehicle not fully in a gear over a long period of time could damage the transmission, automakers developed what is called the "detent system."
The system of detents was often used in conjunction with the "push button" shifters used on many automatics in the 1950s. A detent system uses either a detent spring and ball or a cantilever spring and this spring moves up and down over a series of teethed gears (called a "rooster comb" for how it looks, or an "inner manuel lever") turning the rooster comb to fine center the transmission in the intended gear position at the bottom of each gear.
However, if the spring is too weak to always move the rooster comb to the bottom of the trough between the teeth, the vehicle can be left between gears. On certain Chrysler vehicles the problem is made worse as there is a flat spot between park and reverse where the ball can rest, also resulting in a "false park."
In this video an expert has placed a window into a Ford transmission showing how the transmission can be shifted between gears. The vehicle then moves backwards after a delay.
In this video an expert demonstrates how a driver can inadvertently place a Chrysler transmission with the 'false park" defect between gears. Notice after a few second the the selector self shifts from this dangerous False Park position to Reverse and the car starts moving backward.