This sounds like my daughter! lol- She is just 23 months now, and seems to have just outgrown this. She is a wonderful, happy girl, but for about 3-4 months would just randomly 'bully' other little kids. In fact, for a short time, my sons and I had nicknamed her 'The Baby-bully'! She did not scratch or bite, but would take toys, hit, sit on kids and generally just 'bug' them for no reason. I think she was just testing boundaries, as I would see her look around sometimes before doing these things or she would get this look her eyes, like 'look what I'm going to do'- she wanted the attention. I just kept telling her no, removing her from the situation and telling her 'nice' and doing the face petting thing. Just this week, I have been sooo proud of her and how well she is playing with the other daycare kids and being such a big girl. If I were you, I wouldn't worry, but just keep reinforcing that it is not acceptable behaviour. If it was out of anger, etc. I would be more concerned, but she is likely just curious.