If you are curious to see how this happens mathematically, the idea is that the density wave is proportional to −dξ/dx, and the reflected displacement wave goes like \(\xi_{refl} = −\xi_{inc}(−...If you are curious to see how this happens mathematically, the idea is that the density wave is proportional to −dξ/dx, and the reflected displacement wave goes like ξrefl=−ξinc(−x), where the first minus sign gives the vertical flip and the second the horizontal one.
