In life, things often happen by chance, or for reasons we can't determine. They strike suddenly and unpredictably.
We can write about such things. But overall, a story is not a verbatim record of life. It's a distillation of life. It's edited and organized, arranged to illustrate a particular point or capture a certain idea. The lightning bolt from the blue must, in a story, serve some purpose other than relieving the writer's exhausted imagination.
I suppose one could write an experimental piece in which things happen randomly, to make the point that nothing has a point. But in general, each component of a story has to earn its keep. "Why are you telling me this?" the reader says, or rather, "I assume you have a good reason for telling me this."