Buildup of static electricity. Just like in the high school library. Drag your feet across the carpet then sneak up behind them and touch their ear. Fun times...
Cloth seats are notorious for building a static charge as you slide out.
I simply grab the door as I exit, grounds me and then no shock. Usually happens in the winter, cool dry air passes charge better if I recall correctly. Well not ground per say.
During "shock season" I usually have the habit of holding onto my keys and touching the body with one before I touch the truck myself. At night you can see the little blue arc
Static straps don't do a damn thing. My dad has 2 of those useless things on his truck and you still get shocked bad when you get out. For my parents cars, I get out with my finger in the little pull handle so my fingertip is on the bolt. I never get shocked doing that. On my envoy, I just grab the outside area of the door panel before I start to move out the seat and I am good. I never bother to spray my seats or anything because I cant stand the smell of that junk haha.