A very common reason for Apple to reject an app is because of the Ad SDK that the app is using. Many Ad Network SDKs are not up-to-date with the latest guidelines issued by Apple. (Same applies to Google Play as well).

If you are fed up with your app getting rejected because of the Ad network you are using, check out Avocarrot (http://www.avocarrot.com) for SDKs that are 100% compliant.