Russians now launched an offensive at Kupiansk. That's that northernmost corner.
It would make sense to return to the Oskil river, since it's a better position to defend, plus, if they want Lyman that line would secure the town from the north, so it could serve as a hub for further offensive, and not just a bulge surrounded by the enemy. That is if they manage to capture it. With the current rate of gaining ground this could take a while.
It also makes sense attacking here in the north from logistical point of view. Supplying the forces in Kherson and Zaporozhia on those slim lines which are attacked constantly, must be a pain in the ass. Even for the defensive forces holding the lines. Now cram an attacking force there too, and it's a sure hit for every shell the AFU fires. But here in the north the supply line is short, comes from different directions, air cover is sure.