Whenever a property is accessed that doesn't actually exist, PHP calls the object's __get() method.  If your object is extending \Phalcon\Mvc\Model, you'll need to ensure you call parent::__get() too.  Something like:
public method __get($name){
    if($name == 'arrKeysReversed'){
        $keys = array_keys($this->arr);
        rsort($keys);
        return $keys;
    }
    else{
        return parent::__get($name);
    }
}