Note that the pressure variation leads the displacement by a quarter wavelength.
This is equivalent to
a phase shift of p/2 radians.
When the displacement is maximum or minimum the pressure variation is zero.

Why does pressure variation lead displacement by p/2?

Consider a fluid element

Piston

The element with wave

Volume of the element without wave

Displacement of left side

Displacement of right side

press the Pause/Play button if you want to Pause/Play

The pressure in the element is inversely proportional to the volume of the
element. The bigger the volume, the lower the pressure.

The
volume of the element is largest and the pressure variation is minimum

The
volume of the element is the same as that without wave and the pressure
variation is zero

The
volume of the element is smallest and the pressure variation is maximum

The volume of the element is the same as that
without wave and the pressure variation is zero