My DD did this from about 18 months. She would either bang her head against the wall or push herself backwards and fall to the floor. These were terrifying tatrums and I was always worried about her hurting herself.
I ended up having to surround her with pillows or put her in the cot. I would sit with her and talk calmly with her (yes that is one of the most difficult things to do). Eventually she would start to calm down and slowly move closer to me until she was ready for a hug.
It seemed to work for us because she has stopped doing it now. I couldn't leave her because I really believe she could have done some damage to her little head. She still throws herself to the floor but only pretends to bang her head.
Good luck