I am getting runtime error upon incrementing the pointer, what is the plausible reason ??

I am new to C++, while I was implementing a tag class, I encountered a runtime error. Upon debugging I found out that the runtime error was caused by incrementing pointer(**) attrib_list, but the other pointer which points to same memory address produces no error pon incrementing, Please explain to me what is the reason for this odd behavious ????? I used hackerrank online ide to compile this code

class Tag{
public:
    Tag():
       tagname{""}, sz{0}, t_sz{0}, attrib_list{nullptr}, ptr_nav{nullptr}{
    }

    Tag(string name, int n_att):
        tagname{name}, sz{0}, t_sz{n_att}, attrib_list{new string*[n_att]}{
            for(int i=0; i<n_att; i++){
                attrib_list[i] = new string[2];
            }
            ptr_nav = &attrib_list[0];      //take risk here
    }

    ~Tag(){
        for(int i=0; i< t_sz; i++){
                delete[] attrib_list[i];
            }
        attrib_list = nullptr;
        ptr_nav = nullptr;
        t_sz, sz = 0;
    }


    // this function produces rintime error
    void add_attribute(string name, string value){
        (*attrib_list)[0] = name;
        (*attrib_list)[1] = value; 
        sz++;
        attrib_list++;
    }


  /*
     This does not produce any error, why ????
     void add_attribute(string name, string value){
        (*ptr_nav)[0] = name;
        (*ptr_nav)[1] = value; 
        sz++;
        ptr_nav++;
    }
   */

private:
    string tagname;
    string **attrib_list, **ptr_nav;
    int sz, t_sz;
};

int main() {
    Tag t("tag1", 2);
    t.add_attribute("value1", "1");  //runtime error
    t.add_attribute("value2", "2");  //runtime error
    return 0;
}