J.N.
My Daughter started this at 2. I was so freaked out, she would do it out of no where? This is just like you said a power issue, they think it is pretty col they can create such a response in another human being. I used extreme positive reinforcement. Going over board when she treated kids nicely. "Wow Cass you were gentle and wonderful with so and so I am sooo proud of how nice you were"!! When she pinches try to ignore it, or a simple straight faced that was not nice Cass. and say nothing else. This constant reinforcement only took about a week and it never happened again.
Good Luck!