Accessing vector at index 0 when it's empty

Why I can still get the value from vectors[0] even I already pop_back() at the first step?
I'm guessing now my vectors is empty and check it by calling empty()

#include <iostream>
#include <vector>

struct Vector2 {
    int x, y;

    Vector2(int _x, int _y)
        : x(_x)
        , y(_y) {}

    Vector2(const Vector2& vec)
        : x(vec.x)
        , y(vec.y) {
        std::cout << "[Copy] Vector2{" << x << "," << y << "} copied!"
                  << std::endl;
    }

    void Print() {
        std::cout << "[Print] Vector2{" << x << "," << y << "}" << std::endl;
    }
};

void PrintVectors(const std::vector<Vector2>& vectors) {
    std::cout << "---" << std::endl;
    for (auto& vector : vectors) {
        std::cout << "Vector2{" << vector.x << "," << vector.y << "}"
                  << std::endl;
    }
}

int main() {
    std::vector<Vector2> vectors;
    vectors.push_back({21, 78});

    PrintVectors(vectors);

    // First
    vectors.pop_back();
    if (vectors.empty()) {
        std::cout << "Empty condition!" << std::endl;
    }
    vectors[0].Print(); // ---> I thought I'd get an error at this point.
    vectors.insert(vectors.begin(), {vectors[0].x + 1, vectors[0].y});
    PrintVectors(vectors);

    // Second
    vectors.pop_back();
    vectors[0].Print();
    vectors.insert(vectors.begin(), {vectors[0].x + 1, vectors[0].y});
    PrintVectors(vectors);

    // Third + no pop_back needed
    vectors[0].Print();
    vectors.insert(vectors.begin(), {vectors[0].x + 1, vectors[0].y});
    PrintVectors(vectors);

    // 4th
    vectors.pop_back();
    vectors[0].Print();
    vectors.insert(vectors.begin(), {vectors[0].x + 1, vectors[0].y});
    PrintVectors(vectors);
}

Then, this condition let me debugging through gdb. I see the value of _M_ is changing, but I have no clue what's going on (I can't see the pattern or relation between _M_start and _M_finish).