Short answer: usually not enough resting, or trapped air in the batter.
Cracked tops nearly always mean the shells went into the oven without a proper skin. After piping, rest them at room temperature until dry to a light touch — that skin is what forces the rise down into feet rather than bursting up through the top. Trapped air bubbles do it too, so tap the trays firmly on the worktop after piping to knock them out. An oven that’s too hot can also crack them, so an oven thermometer is worth having.
Related:raspberry French macarons · glossary:macaron feet,macaronage.