I'm writing a story that happens on a dry and arid planet that has only a few oasis's where life can thrive. Life on this world either lives in the oases or lurks in the sand waiting for migrating animals to pass by. The inhabitants of the world are also nomadic, spending some time at each location before moving on so as to not burden the land. Though there are exceptions of course, some groups prefer to settle in a single oasis and expand it. However this world wasn't always a desert, otherwise life as we know wouldn't be able to develop. The planet was once covered in oceans and something caused it to dry out.
What could cause oceans to disappear? Either through a cataclysm, though life may not survive or through some long term change in the environment.
I'm not too late in the writing process, so there's wiggle room for explanations. Although it has to allow for life to survive the drying, so no meteors evaporating the oceans or anything that extreme. My first guess was for the seas to get sucked under the planets crust, though I am no seismologist so I don't know the specifics of how that would happen.