A knight on the back rank next to its cornered king has too few squares. The attacking king steps in, the rook cuts the knight's escape, and the knight is won or the king mated.
The knight is two squares from its king, so Black might think the position is safe. But the pieces are on the edge: the king on g8 and the knight on e8 have only a handful of squares, and White's king on e6 is already close.