It is probably something that she has discovered that provokes a reaction and therefore an audience to her tantrum. i can't imagine that she will do herself more than superficial harm. The self preservation instinct would be too strong for that and if she really hurt herself, she would stop doing it. My son used to bang his head on the ground when he had a tantrum - even if he was out on the pavement. He soon stopped doing it when we walked away and left him to it! But the first couple of times, your instinct is to rush to him and get him to stop. That doesn't work.