My daugher (21 months now) did and still sometimes does this. She is and was very verbal, so no delays there. She just gets frustrated easily and does not know how to expres sit. She does not hurt herself, but she noticed that it shocked and aggravated me to no end. Ignoring it and redirecting her helped. I acknowledged her frustration and showed her appropriate things that she could hit or bite. I see you are mad, you can bite your blankie, but not your hand or people. Here, hit the chair when you are frustrated. It seemed to work, but the real motivation behind it was my reaction. Good luck - this too shall pass.