Why do Euler's angles and y-axis have a negative sign?

[LuisOrtizF]

Could someone explain to me why the IMU measurements on the y-axis are sent with a gegative sign to the Madgwick filter. And why for the visualization the Euler angles are sent are also a negative sign?

imu.updateMadgwickQuaternion(gx * PI / 180.0f , -gy * PI / 180.0f , gz * PI / 180.0f , ax, -ay, az, -mx, -my, mz);

    Serial.print("Orientation: ");
    Serial.print(-heading);
    Serial.print(" ");
    Serial.print(-pitch);
    Serial.print(" ");
    Serial.println(-roll);