A reason for assenting in the mind is the evidential middle term through which we judge a conclusion to follow.
A cause in reality is what actually accounts for the property being instantiated in the subject under discussion. The first is an epistemic role; the second is an explanatory or causal role.
Seeing water drops on a wall may give us evidence that it has become wet. Our seeing the drops did not wet the wall. The reason we come to know something need not be what makes it happen.