This kind of behaviour often does surface at about the age you are talking about and it often has to do with the father or other male figure in the child's life starting to rough house with them in the evenings. The child just doesn't understand how hard they doing the behaviours as the adult can likely take it. Sometimes you can do a that is an at home behaviour and not a daycare behaviour and they will learn to stop with a reminder. Try asking the child where they learned it.If they say dad then speak to the parent about the rough housing and how the child does not know the difference and as a result is now a bully in daycare. Take a look at the child's face and demeaner while they are doing the agresssion in the sense of do they look like they are trying to play or do they have an I am angry kind of look and being very agressive.