Macarons crack in the oven for a variety of reasons, the most common being the oven itself. For perfect chocolate macarons, you need your meringue and batter whipped and mixed to the right consistency. And then a gentle, even heat in the oven. Most home ovens have hot spots that can force the air in the meringue up through the top of the shells before they have time to set during baking. And then... crack! Often you will find a batch of macarons on the same pan produce a nice portion of great-looking shells and others that are cracked. Investing in an oven thermometer can help you correct any trouble with temperature, and also find hot spots to avoid.