Russian soldiers found the remains of Soviet soldier while digging trenches in Nova Khakovka

While preparing for defense, Russian soldiers, digging trenches to strengthen their reserve positions in the area of Nova Kakhovka, came across the remains of a Soviet soldier from the Great Patriotic War.

If you look closely at the video, you can see that many trenches are being dug around the city.
Why trenches if the army is preparing to retreat?