That one Colonel MacGregor had a very intelligent analysis of the situation.
While dropping back, Russia has crippled Ukraine's ability to bring numbers forward quickly (taking out supply chains, electric grid, and trains), so smaller numbers are advancing forward and getting wiped out. He also says it has to do with the terrain, that it's so flat and open that they're dropping back into a more defensible position and will be waiting it out.
Several interviewees have stated that it is a well-known Russian tactic (studied in their military schools) to drop back in order to draw the enemy in where they want them.
Problem is that the Western media is so hostile to Russia, that every time Russians leave a village, it's front-page news that there's some massive Ukrainian counter-offensive, so it's hard to know exactly what's going on. But MacGregor seems like he has reliable information from sources still in the US military.