It's happened to me on two occasions.
One time when I was about twelve or thirteen, I was writing a book I never ended up finishing. It wasn't great, looking back on it, but it was the first time in a while I had been really passionate about a project. It was a story about a twelve year old boy who's sister had died when he was young, and he had been imagining her ever since in order to cope. It was meant to develop into more of a horror-story type thing about the boy trying to get his 'sister' to go away and how she turned into something evil that started to kill him slowly, blah blah blah etc. But to begin with, it was more tragic than scary.
My parents got me to read it out to one of their friends who was visited. I don't remember what part it was exactly, but by the time I stopped reading the chapter, I looked up and she had tears in her eyes. In that chapter, the fact about the sister being dead was kind of ambiguous. I asked her what she thought was happening, and her voice cracked when she said: "The sister's dead, isn't she?"
It really freaked me out and I didn't know how to react, because I knew why she was crying. I'm guessing it was because that woman's brother had died in the past year. I just felt guilty about it.
The other time was more recent, when a short story of mine got published in the newspaper. It was a story about two boys who shoot a bird with a slingshot only to regret it immediately and try to heal the bird. They try to keep it safe, but after tie passes they decide to let it go. But they let it go by the river where it flies into the wall on the other side. A friend of my parent's emailed me to say she read the story on the bus and cried because of it. I had a better reaction than the time I had a few years beforehand, as I felt this wasn't me tapping into some hidden wound in the person. Like I earned the cry, if that makes sense? I don't know.
Source: reddit post