my 2 year old isn't doing it (yet!), but i know ds did at around 2.3/3ish.
i wonder if it's frustration in not being able to get their point across, they're learning to be independant, but don't always have a) the skills to communicate what they want to do or b) the skills to actually do it.
i'd wait it out, try not to react too much, maybe just say 'please don't hurt mummy, it makes mummy sad' (sounds cheesy, worked with mine though), and hopefully once they gain more skills etc, the frustration will peter out.
i'm sure it's all completely normal xo