My son used to do this all of the time, including hitting other children for no apparent reason, and it turned out that majority of the behaviour was due to him having chronic ear infections, and he was having difficulty hearing and developing his speech. It took a year of ear infections before finally getting a referral to a ENT surgeon - where she decided he needed to have gromits put in and adnoids taken out the next week - even though I wad 8 and a half months preggo with no 2.

The change in his behaviour and speech was basically overnight, although we still have plenty of 2 yo tanties, with tears but no self harming behaviour.