As an at will contract employee does the employer have to give a reason to end the contract and does it have to be reasonable?

The employer in an at will context does not need any reason to fire someone. If there is a reason, it need not be rational.  An employee can be terminated for a wrong reason, or based on mistaken facts.  None of that matters, as long as the termination isn't for an impermissible reason, such as being based on sex, age, ethnic origin, marital status, religion, etc.  In that case, the employer does have liability.  Also, sometimes there is an implied contract that makes the situation not truly "at will," such as where there is a promise of employment for a period of time as an inducement to move.  The employee in most situations is at the mercy of the employer.  This explains why the people in management who make these decisions usually negotiate their severance terms as a condition of employment.  It may not seem fair, but it is just another instance in which the law favors the business over the person. (The converse is nearly mythic.) The belief is that this allows businesses to prosper and that we all benefit as a result.