There is no motive. Its stated in the story that the man and the young girl have no motive for following the old couple. Neither of them know why they are doing it. Its just something that happened. Like life. They were bored or had nothing else to do or whatever and just decided to follow these people and just happened to witness the couple being told that their son is dead. There isn't always a motive for something. Or maybe you could say they were motivated by their boredom to follow these people. Or maybe there was something about the couple that that intrigued them. The girl says in the very beginning that to her they seemed very unalike, so maybe they decided to follow them because of how "not alike" they were. The end is the end. You are reading a story about two people following two other people with no expectation of anything exciting happening and then they are shocked at the scene they come upon and the news that this couple receives. Are you saying that i should have a motive as a writer as to why i've chosen to have the story end this way? For one, i didn't think this story out. It just came and i made it up as i went along. I just wrote an odd story with an odd ending and i like it.
Read Shirley Jackson. Read Flannery O'Connor. By no means am i comparing myself to either of them, nor am i saying that my story is great or even very good for that matter. But the sense that one gets at the end of my story is similar to that felt by a reader at the end of one of theirs.
I do appreciate you input though, and i hope that you don't think i'm offended at all by what you've said. I just don't agree with there having to be a motive on my part or on the part of my characters.