METARs suggest that weather conditions were deteriorating when the accident occured. Around sunset (~4 hours later), rain was reported in the area.
Under these conditions, it is not uncommon for large amounts of clouds & mist to build up along mountainsides within minutes (especially in a densely wooded area that is able to contain lots of moisture). Such highly terrain dependent weather phenomenae can hardly be reflected in METARs or the like.
To date, we can only speculate if the eastern slope of the "canyon" they entered was hidden/covered by some local weather phenomenon, therefore leading to misjudgement of position and heading.
Another possibility could be distraction ("VIP in cockpit").
As soon as FDR & CVR data is read out, we will know... hopefully.
