The term flyaway is pretty loosely used for any control loss be it user or craft that causes the issues, there have been a few posts by people that had flyway's and of these after recovery it has generally shown the issues were either prop falling off or other more simple explanations. Some are still unexplainable even after recovery but there are so many possible variables that failure of any single component could have been the cause. There is no data logging and if it never comes back its impossible to say what causes this but you need to remember that people in forums tend to use the term flyaway too loosely.
Also if you look at some of the new user posts and how little many know about what they are doing it explains quite a large number of so called flyaways. How many first posts are here with people saying their new phantom doesnt fly as they dont know how to engage motors, so many have no idea what the LED lights mean and so many dont understand the limitation of GPS.
You cant rule out firmware glitches but as flyaways are still very rare considering how many phantom's and DJI controllers are actually being flown, I dont see it as a huge issue. It has yet to be proven there is any fault that causes a flyaway in a repeatable way as that would actually be fixable by DJI if it was. Most issues I think are caused by pilot error, radio interference, GPS or calibration issues etc not likely a specific glitch or bug in the controller or firmware.
People have been losing and crashing RC aircraft for many years, it has just got more popular with the new quads like the phantom. They are very simple to fly for anyone but a simple thing like losing GPS will switch it into atti mode and many new users will lose control in this mode and call it a flyaway. There are experienced people that have had issues too as its not just a beginner issues and hardware/software is never perfect and anything may fail so every flight is always a risk. This is why you need to make sure you get to know all about every mode/option/calibration on the phantom along with good maintenance to give you the best chance of making a safe landing after every flight.