Ugh, I hated it. It was very upsetting and he did it for many months! Once his words came in better and he learned the sign for "help" it got better but still had some anger issues. Our pedi said it is common and 5x more likely with boys! I was always worried too about injury but nothing. Most of time it is for attention/frustration and he only did it once/twice in a row, not continuious. I started to ignore it and walk away, he would then chase me and cry and grab at me etc.
McBoy also had a prostetic helmet for a LONG time so all this was after that was off and of course we threatened to put it back on him and he would run from it! haha. Not everyone has that convenience.
Try and teach him the sign or to say Help. And I found that if I had sympathy, he felt his point was made and didn't need to keep doing it. "I am sorry you are made, lets find another toy" and hand him something. McBoy it was often at food times, didnt like something cut a certain way or the crust was a bit detached from toast (very OCD). Etc. YOu will find patterns and deal with it just fine! IN the meantime he is ok! Its harder for us to watch.