Common practice for retreating forces. There are in excess of 1 millions mines in the DMZ between North and South Korea, war is hell folks.
The article makes the point that large numbers of mines were in farmer's fields with no military value.
Considering Putin has been bombing: shopping malls, food stores, and actively stealing large amounts of Wheat, it's clear that Russia's aim is to damage food production in the country.
I believe Russia thinks it might gain some advantage, perhaps even crush the west, by worsening hyperinflation across the world by hampering agricultural exports from Ukraine. Medvedev said something like inflation will crush the US recently.
It won't, at least not because of Russia, because India and other countries are already exporting record amounts of agriculture in response due to market forces.
That said, I expect inflation to only get worse in the USA due to spending and dumb Covid policies still in effect at our major ports.
It won't kill us though. It'll just put Trump back in the White House where we will reverse these policies.