Two things I've heard... one was that the pilot wasn't instrument rated. Two that the Bangor Navy base reported a squall in the area at the time. 

If this is correct, and given the apparent near vertical impact based on the debris, a pilot's guess would be that he lost contact with the ground and horizon and spun in. A very easy thing to do for those not instrument rated. There are innumerable accidents of this type over the history of aviation. 

Anyway, this is informed speculation and the eventual accident report will reveal the truth. But until then, it's another caution to all VFR pilots that trying to duck under lowering clouds is hazardous, especially if there are squalls or convective clouds in the area. Not a big issue in summer in the PNW, but in winter, the clouds can quickly drop to ground level, especially over rising terrain.