two exogenous causes, in ranked order: (1) Steve Ballmer's Win 8, 8.1
This is exaggerated. Apple laptop sales are declining just as badly as windows laptop sales.
The economy has been sub-par for 5 years now.
Emerging market PC growth is clearly stunted by cheaper computing devices like tablets/phablets/smartphones. This used to be a major growth driver for Intel and somewhat explains Intel's desperation to break into this market with cheap/subsidized Baytrails.