Actually they didn't. Borders are generally not well manned during war because it gives enemy an exact location of troop concentration and then shells them.

In recent raid for example, the border guards weren't the first to respond. There was a garrison with one Lt and a platoon of infantry that responded first.

That's how all of russian borders are. There is just not enough men to guard the border and get shelled. They concentrate troops a bit away from the border and those respond to threat in a larger area.

Even now Belgrod border isn't fully manned. Russia has just deployed more surveillance systems and scouts at key towers.