Why do autistic children approach strangers so openly, and why do they randomly hit others?

To be very honest, I don't know much about autism, and was hoping to be enlightened. I was at a park today, reading a book, minding my own when an autistic child ( about 13 yrs old) walked up to me and started rubbing my arm. I was kind of taken aback, as one might imagine. He seemed friendly enough, and all of a sudden BAM! --he punches be square in the eye, knocking off my glasses. His mother who was probably 20 yards away ran up, said a simple "sorry" and walked off dragging the child by the arm. What causes autistic children to openly approach strangers, and why do they perform random acts of....blatant violence? ( I'm sure they don't mean to be, just curious) Answer this and get my "best question" vote!