So Earlier this morning i was flying the DJI Spark that i just recently bought and i gotta say, i think the sphere panorama shot is absolutley amazing. So I decided to take the drone over the lake a little bit and have the panorama take the shot. Halfway through the shooting my phone crashes due to apple screwing me over the years and a temperature of about 40 F . i had visual of the drone however, it was probably about 250m out so it was a dot. Iv'e read a few stories about fly aways so i am a very cautious pilot when i do fly. I calibrate every flight and make sure i always see the drone, My only fear was that it would try to autoland and go into the lake. I had never had to use return to home before but i always set it where theres no metal around and open space. I pressed the button and heard the beeping and shortly after saw my drone speeding in at 100 ft or so and land only about a meter off max of where it took off. everything worked great, Im just more or less curious what does cause fly aways and if there are any other steps to prevent them? Thanks

Yes! The sphere panorama is great, isn't it?

While some of the flyaways I've read of seem to be caused by a hardware issue, others have been pilot issues.  

I had a close call similar to yours where my iPhone died on me. I had about a 30% charge left  on it and thought I had enough power to take the Spark up for one more photo. I was wrong. Outside temperature was in the low 40s. Suddenly my phone's screen went black and I saw that it was powering off. Fortunately my Spark was not very far away and I was able to bring it back in using just the controller. Having practiced flying visually indoors this past winter certainly prevented me from panicking and paid off in my situation. LOL.

It sounds to me like you are pretty vigilant and based on the video below, keeping the aircraft within line of sight like you were doing is key to not losing your drone. Also, thoroughly scouting the area in which you are flying is helpful in case you need to do an emergency landing. The video is about 15 minutes long and there's some excellent info in there on how to not lose your drone.

The drone will halt GPS aided position stabilization anytime the compass swings funny due to interferrence at take off or during the flight.  It then goes into ATTI mode and the Spark relies on the pilot/photographer to keep the drone in a stabilized hover or on its flight path.

Not being able to fly a quadcopter in ATTI mode causes fly aways.  Now if that means the pilot can't handle ATTI mode right near him or if he has skills but has flown it so far out that it is a vague dot, his inability to fly in ATTI mode causes the drone to fly away.  The stabilization provided by the Spark's IMU keeps it from crashing immediately, which would be the normal "lack of pilot skills" result.  Instead, the IMU keeps the drone somewhat level but not level enough to keep it from drifting away (+winds?) without constant pilot correction.

The RC model aircraft of old were self correcting in that as soon as the pilot got it so far away that it was a dot, it would crash because he couldn't provide accurate control inputs.  Couldn't control what he couldn't see well enough to tell which way it was facing.  The plane would crash right where it was and woudn't fly away.  Pilots like me learned to keep the model relatively close.  The Spark's IMU keeps it at altitude and level so it flies away, instead of crashing, when it doesn't have GPS or pilot corrections.

Oh yeah, and winds.  Winds have taken plenty of models over the years.

Great info! i understand that while alot happens with the tech, Most of the time its pilot error and so this is great info to know! i recently downloaded the Hover app and started learning more info such as the KP Index. along with winds and weather, Understanding magnetic interference can prepare you for expecting slight interference and prepare you to be more cautious, thanks for yalls time!