It's a stage most kids go through. My mom had a daycare when I was younger & I basically saw the stages a kid goes through. Terrible twos are what people call it (not that the kid is terrible), eventually the kid will get over it.
When I was a little dude, acting-up, playin' da fool . . . an arm would appear out of thin air and hoist me by my ear into another universe . . . then it would drop me in front of my mom or grandmother who would explain in no uncertain terms that my actions were grounds for discipline by the Head of the Universe (and I wouldn't like it) if I continued to be a Spoonhead.
They are constantly pushing boundaries. Around this age I would imagine they are not only discovering they can actually do these things; but, they are learning they get a reaction from it when they do. It's possible and could be a combo of exploration and mimicking behavior. I found stopping it early and being careful not to over react helpful with my son. A lot kids are rough and need to be taught to be more gentle. Also, they are emotionally not developed enough to discern from yours, mine and someone else's property OR the whole 'sharing' concept - not to mention that they are not fully able to communicate their thoughts, ideas and feelings properly. So acting out physically is easy for them. It's normal and kids get through it. If it's an anger thing, help them find a better outlet and step in to teach it.