The version I heard back in the day was that the door could be opened at speed and it would blow off, or at least be flattened back allowing a person to jump from a moving vehicle.
I always thought it was if you were standing in front of the open suicide door and the driver took off you would be flattened by the door - versus the standard front hinged door where you would just be left standing there.
Of course if the driver was backing up your mileage may differ.