Reason refers to causes that operate through medium of judgment.
I need money, so I get a job. I made a judgment (‘I must get a job to solve my money problems’), and that judgment is the cause of my subsequent action. Here we have a case of reason as cause.
One rock collides with another. The second one moves. The collision is the cause of the movement, but neither makes a judgment as to what it must do, and neither rock therefore has any reason for doing what it does.
One can speak of the reason the rock moved, but not of the rock’s reason for doing so. By contrast, one can speak both of the reason I got a job as well as my reason for doing so—and those two reasons (setting aside cases of self-deception on my part) must coincide.