Why is Thomas Hardy considered a naturalist? What is an example of chance or coincidence in Tess of the d'Urbervilles?

Naturalism as a literary movement is distinct from Romanticism or Realism in that it takes a detached, dispassionate view of human life. As on naturalism puts it:

Naturalism applies both to scientific ideas and principles, such as instinct and Darwin’s theory of evolution, and to fiction. Authors in this movement wrote stories in which the characters behave in accordance with the impulses and drives of animals in nature.

This doesn't mean that the characters act like animals, but rather that the relationship of characters to nature is like that of animals. Unlike Romanticism, where the human subject strives to transcend or subdue nature, or realism, which often is concerned with the interior life of characters, naturalist authors set their stories going and then coolly observe what might happen, much like a naturalist studies a new species. The world of a naturalist novel generally is amoral. Bad things happen, as Tess amply shows, but causes of these events are often random and the author's attitude toward them is remote and clinical.

A perfect example of this principle at work in Tess is the event that sets the entire plot in motion. In Chapter one, Tess's father, John Durbeyfield, is met on the road by Parson Tringham, who addresses him as "Sir John" as a joke and, in so doing, sets the whole story in motion. John asks him why he addresses him so, and it comes out that the parson has discovered that the Durbeyfields are the last remnant of the ancient d'Urberville family. This new connection causes Tess's parents to hatch a scheme to send Tess to visit a family they think are wealthy relatives, but actually are not connected at all. Tess meets Alec, and her life is changed forever.

The coincidence of Durbeyfield and the parson happening to meet, and the parson being in a mischievous frame of mind, and happening to know this thing about John's family, causes serious consequences for Tess, but there is nothing explicitly moral or fated about this event. It is purely an accident, just like Angel just missing a dance with Tess in Chapter Two is an accident, or the family horse being killed in Chapter Three. Whatever moral judgements Hardy's characters make, they are always subordinate to chance. Hardy's point is that that the world is filled with coincidences, and the choices we make in responding to them often are based on nothing more cosmic than wanting a bed for the night or a hot meal.