reason: why something happened (often motive)
cause: how something happened (to effect, bring about)
What's the difference between i-yu and weon-in?.
Reason and cause are often used interchangeably, but there are situations in which reason should not be used instead of cause.
An example of when either reason or cause can be used: There
is a reason for his anger.
An example of when reason would not replace cause: What was the reason that the snow melted so quickly?