I always assumed it was just coincidence that Willoughby happened to be in the vicinity when Marianne fell down the hill. After all, he appeared to be hunting, and he was used to the cottage being unoccupied. I also assumed the fall took place some distance from the house.

However, Chapter 9 tells us...
"One consolation however remained for them, to which the exigence of the moment gave more than usual propriety; it was that of running with all possible speed down the steep side of the hill which led immediately to their garden gate.

They set off. Marianne had at first the advantage, but a false step brought her suddenly to the ground, and Margaret, unable to stop herself to assist her, was involuntarily hurried along, and reached the bottom in safety.

A gentleman carrying a gun, with two pointers playing round him, was passing up the hill and within a few yards of Marianne, when her accident happened.

So the accident actually took place in fairly close proximity to Barton Cottage and it appears Willoughby was practically on top of them when it happened. OK, he was in that location because he assumed the cottage to be unoccupied.

Except, he tells us in Chapter 14

"How often did I wish," added he, "when I was at Allenham this time twelvemonth, that Barton Cottage were inhabited! I never passed within view of it without admiring its situation, and grieving that no one should live in it. How little did I then think that the very first news I should hear from Mrs. Smith, when I next came into the country, would be that Barton cottage was taken! and I felt an immediate satisfaction and interest in the event, which nothing but a kind of prescience of what happiness I should experience from it, can account for.

Wouldn't it be inappropriate for Willoughby to be hanging around the cottage when he knew it was occupied, and by people to whom he had not been introduced? After all, if he was curious about meeting the new tenants, he could always count on Sir John to provide an introduction.