I would say it's the cloth seats. I think there is something you can spray on them to prevent it, but I can't recall what it is.
I agree, it's the cloth seats. The spray is called "Static Guard". My 2007 F150 shocks me every time the humidity is low. I use it every winter and it works really well. Buy Anti-Static Spray - Eliminate Static Cling -Static Guard
2014 Ruby Red V6 Premium w/pony package, Magnaflow Street axle-backs, LED license plate light, LED interior lights, Strip LED trunk lights, MMD Hood struts, SCT X4 Tuner with Steeda tunes for life, Boss 302 strut tower brace, OEM extended range remote start, Raxiom Navigation.