The term is used to describe the nature of one freaking out or acting worried as if it were a chronic disease that they acquired as a young child.
The word sketch comes from someone freaking out or acting strange or worried.
I.E. Dude, why are you sketching out?
Translation: Dude, why are you freaking out?
Kid 1: Dude, I'm freaking out; I don't feel safe siting here in this car, there might be police around
kid 2: What's wrong man you got the sketch-pits?