The quick action has no user interaction. As soon as component is initialized, it runs the init method which calls server side controller.
This is good when the case record page loads for the first time and user clicks on "Assign Case" quick action. The component is initialized which runs the code.
Now if user moves away from "Assign Case" quick action (lets say clicks on some other quick action) and comes back to "Assign case", i have to assume that user whats my server side code to run again. But since the lightning component is already initialized, code doesn't run again.
In short, i want "Assign case" quick action to behave like a button. Every time user clicks on it, component should initialized again, which would run the code in init function.