Oceanic 815 had mid-air break-up due to electromagnetic pull. This was triggered by Desmond's failure to push the button on 22 September, 2004, the same day as the crash. This resulted in a System Failure that involved an intense magnetic field from behind the concrete wall in the Swan. By the time Desmond was able to enter the numbers, the airframe of Oceanic 815 had undergone such violent stress that it disintegrated.
In "Further Instructions", we see that the discharge has caused to the Swan to implode into a deep, featureless, conical hole. This demonstrates that the forces involved in the Swan are extremely powerful.
From the Barracks vantage point, the break-up seems strangely surgical, as if the fuselage had been ripped by some exterior force. The black bands that appear around the points of seperation are probably metal fatigue marks caused by stress.
First, Oceanic 815 loses its radio. Then it experiences increasing turbulence, descends and breaks up. These behaviors seem compatible with being under the growing influence of an electromagnetic field that acts like a "tractor-beam."
Except for the right engine, the only evidence of explosions are booming noises. If the first piece seen flying off the airframe is the forward cargo door, it appears intact, even undamaged, as if it had popped off due to a failure of its latch caused by stress, rather than an explosion in the cargo hold.
A Boeing 777 is 11% ferrous. This means that it can be pulled by an electromagentic field. Because most of the aircraft is not ferrous, such a pull would cause uneven stress on the airframe, perhaps contributing to its break-up.
It may be the intent of the writers/producers that all the problems with this theory are explained away by the single word "unique" in the phrase "unique electromagnetic fluctuations."
A Boeing 777 is 89% nonferrous, so the electromagentic field would have to be that much stronger to pull on the aircraft.
Unless the electromagnetic field was somehow focused, the distance of the aircraft at cruise altitude would require such an immensely powerful electromagnetic field that every piece of ferrous metal on the island would have been been at least distorted, probably ripped apart. Also, the electromagnetic pulse from such an event would probably have damaged or destroyed all electronics on the island. While there was some reaction by metal during the system failures and the discharge, the effects were not of this magnitude, and some electronic gear survived all these events.
Furthermore, it is hard to believe that the electromagnetism being built up can go from being safe to being able to pull down a plane, only a few minutes after the 108 minute threshold.