Impeaching a president is a really big deal that has huge consequences for the nation. So the framers wanted to make sure that impeaching the president could only be done when there was an overwhelmingly convincing reason to do so. Your best answer is the last one: the framers wanted to ensure that presidents were not removed from office unfairly.
The farmers often believed that if the process was made more difficult and drawn out it would be harder to violate the laws and citizens rights and liberties.