If I remember correctly, that happens because when on calorific deficit, by default people reduce their activity - this is the instinctual response. Body respon

If I remember correctly, that happens because when on calorific deficit, by default people reduce their activity - this is the instinctual response. Body responds by losing muscular mass as energetically it is costlier to maintain than fat. This is why it is important to maintain or even increase the activity while losing weight.