On a rolling shutter camera (like the ones on DJI drones and GoPros) each line gets exposed at one time and then read out of the sensor. Then the next. Then the next... From top to bottom.
If you are flying forward at 10 m/s at a height of 40 m, your spot length is 51.86 meters and at 30 fps, the frame time is 33 ms. In 33 ms you travel .33 meters. That means that the distance from the first line that is exposed to the last line of the frame being exposed is 51.53 meters or a .6% difference in length. That is more than enough to mess up a lot of reconstruction calculations.
Numbers are from www.mapsmadeeasy.com/flight_planner
This effect shouldn't cause any horizontal distortions in the images since the change occurs along the path of travel.
Previously, when our app was flying forwards on the way out and backwards on the way back, we were getting some really weird calibration results due to the fact that we were compressing the images linearly on the way out and expanding them on the way back. We have changed the flight planning now so at least the badness is consistent.
If you really need accurate data with a DJI aircraft, you will need to fly pretty slow.