This happens for a few reasons. 1. You opened the oven too soon. Whenever the oven is opened you are cooling it down. The top of the cheesecake is very sensitive to temperature fluxuations and does not like sudden changes before it is ready. The cool air entering the oven cools the top of the cheesecake too quickly and causes the cracking. 2. Overstirring the batter. Any stirring will incorporate a little bit of air into the batter and this is ok to a certain extent. Overstirring will allow too much air into the batter and create bubbles that get trapped. Bubbles will pop during baking which causes cracking. 3. Overbaking the cheesecake can also cause the top to crack.