Tukukkk

My world has a semi-nomadic people that inhabit a vast sandy desert. Underneath this desert lie the ruins of an enormous metropolis -- a city that was abandoned tens of thousands of years ago.

In the past fifteen to twenty centuries something has changed in the region which has resulted in the slow uncovering of these long-lost buildings. This process could have been going on for longer, but only in the past few centuries have the ruins actually started to surface.

What is the slow, gradual change that started this? My first thought was a shifting of the direction of the prevailing winds, causing sand to be blown away instead of deposited, but I don't know how realistic this is.

This happens all the time over a relatively short timescale. Sand travels across the desert in wind-driven waves - these are usually described as dunes but they move.

A building could be covered one day by the crest of a wave and a week later be exposed in the trough of the wave.

I imagine that originally the area was a vast fertile plain and underneath the soil was alluvial rock. When the plain started to become desert, the sand gradually blew over it over hundreds or thousands of years, eventually burying the city. As the wind continues to drive the sand along, eventually the bedrock is uncovered. The foundations of the buildings were cemented to it and so they appear gradually. There is no need to change the direction of the prevailing wind.

Before the metropolis was covered by the desert, it was drowned in a sea.

https://www.slowfood.com/a-salt-storm-from-the-aral-sea-save-the-planet-from-climate-change/

I am thinking of H.P. Lovecraft's Sarnath, claimed by its lake.

These men indeed went to the lake to view Sarnath; but though they
found the vast still lake itself, and the grey rock Akurion which
rears high above it near the shore, they beheld not the wonder of the
world and pride of all mankind. Where once had risen walls of 300
cubits and towers yet higher, now stretched only the marshy shore, and
where once had dwelt fifty millions of men now crawled only the
detestable green water-lizard. Not even the mines of precious metal
remained, for DOOM had come to Sarnath.
The Doom that came to Sarnath

A sea claimed the metropolis, covering it in the waters. Over the millennia silt and sand built up, burying the sunken city. Years passed; the world changed. A thousand years ago the sea dried up, leaving only an expanse of salt and sand.

Winds and rare rains then eroded the mudstone into peaks and spires. Like an ancient fossil, the entombed city gradually emerged into the light.
https://en.wikipedia.org/wiki/Badlands#/media/File:Badlands00503.JPG