I want to ease your mind and say that Pavlova will crack most often than not. This happens due to various reasons, but most likely it's the overbeating meringue that causes cracking because it puffs up a lot while baking and then deflates while cooling. Other than that, the too-high oven temperature can also cause cracking, because the outside gets dry too quickly and thus overbakes or you didn't let the meringue cool gradually in the oven. The sudden change of temperature shocks the meringue and causes it to crack and collapse.