What makes the ending of “The Interlopers” surprising? Has Saki prepared readers for the ending, or has he led readers to expect a different kind of ending?

Two things that make the ending surprising are its suddenness and tragic irony. Saki prepares the way for this ending by painting a picture early on of the wildness in the forest that night and by describing how, even though the two men hate each other, they can't kill each other when "face to face."
The ending is surprising—even shocking—because it is so sudden. The ending is comprised of one single spoken (not yelled) word: "Wolves." This one word is preceded by only a single word, "No," accompanied by wildly fearful laughter full of irony, "the idiotic chattering laugh of a man unstrung with hideous fear."
The ending is shockingly surprising also because it ironically follows such peacemaking. In the presence of each other's suffering and courage, the men end their feud and decide they'll be friends and share hospitality; they'll share hospitality and friendliness, concern, and care. Each pledges to himself and the other that when his men come to the rescue, they shall free the other trapped man first; they shall save the new friend before the master.

"Neighbour," [Ulrich] said presently. . . "If my men are the first to come you shall be the first to be helped, as though you were my guest. We have quarrelled like devils all our lives. . . I've come to think we've been rather fools; there are better things in life than getting the better of a boundary dispute."

Saki prepares the way for this ending—he does not trick us with an unprepared (or ill-prepared) ending. Early on, he establishes that it is a wild night among the animals of the forest on this stormy "wind-scourged winter night," unnaturally disturbed by poachers, thereby paving the way for the sudden appearance of the wolves. Saki prepares us for encountering an ironic twist in the ending through presenting the double irony of having enemies trapped beneath a beech branch—held helplessly together—when moments before they had been "man to man" and "face to face," each intent on the death of the other yet unable to kill.

The two enemies stood glaring at one another for a long silent moment. Each had a rifle in his hand, each had hate in his heart and murder uppermost in his mind. . . [A] man who has been brought up under the code of a restraining civilisation cannot easily nerve himself to shoot down his neighbour in cold blood.

Saki also prepares us by presenting the ironic twist that, having mended their feud and become friends, they are still trapped together and awaiting either rescue or death: rescue, if their men find them, death beneath the branch in winter cold if they do not. It does not occur to the men to consider the ironic twist to their death Saki has prepared for them: wild wolves.