fly aways occur for a few reason.
1. a signal superior to that of the TX over takes the phantom (this issue has been fixed with firmware updates)
2. the pre flight checklist of things were not done (most common is the compass calibration, followed by not getting enough sats)
3. the most common reason for a flyaway where the unit heads in one direction and takes off, a motor issue. The Naza is designed to keep the phantom balanced and steady. If a motor has an issue that causes it to spin slower, the Naza will try and over come this issue with more power and/or less power on other motors. In some cases the Naza is not able to save it and it flights the bad motor which in turn causes the unit to "take off". Picture a drunk guy walking down the road, he attempts to walk forward in a straight line but he starts tipping over. His body naturally tries to fix this issue by increasing his speed in that direction to help get his feet under his head. Instead of him leveling out, he has a flyaway into whatever object is off at a 2 o'clock angle from where he was headed.
4. In the case of this video:
https://www.youtube.com/watch?v=HkQ9eB7M7iQ the pilot is flying in an area of heavy disturbance. Watch this whole video, he is next to train tracks, a never ending set of power poles, and commercial buildings with god knows what going on inside. Any one of those buildings could house equipment that runs on a 5.8ghz system and is filling and clouding the channels.
5. My personal favorite, putting the lid on wrong. If you have flown a 450/550 or any DIY setup with a Naza, you know that if the GPS puck and the Naza are not pointing in the same direction, you are going to have a huge issue. It will not hold GPS lock and will occasionally just start to head off in a random direction. The same would happen with the Phantom. It will fly perfectly in ATT mode but the second you put it in GPS, its heading off to no mans land.
6. Compass placement. See #5. If the compass is not put on the correct leg, your calibration is not going to be correct and it will not be able to return to home or function correctly.
7. Crashes/unbalanced props/drops/misuse. The RX can disconnect if the unit suffered a hard enough hit. unbalanced props can vibrate the unit which can also disconnect the RX. If it disconnects, the unit should land on its own but could instead just fly away.
8. bad pilots. A bad pilot can also cause #2-7. Neglect as well as ignorance is not an excuse. if you are not checking your unit careful before all flights, don't be surprised if you don't have a phantom anymore.