Bad piloting. You know it just takes a click of a few buttons on a lot of those things right? Unless the pilot could see it with his eyeballs and had a controller, in which case it hardly merits using a drone in the first place .
Here would be a good example, I strap a brick to my quadcopter, set a route with my auto pilot software, set an altitude of 1500 ft. Click go. It will go up to 1500ft, fly to location and land.