Just to be clear, for any LEGAL reason. "At-will" in no way negates statutory rights such as anti-discrimination law or refusal to commit illegal acts. Retaliation against certain protected acts is forbidden. Also, like almost everything under state law there is significant variety even within a given category. There are many states which are at-will overall but also have state level exceptions for things like public policy (this is where whistle blowing can fall under, though that involves some complicated questions too), implied covenants, etc. Finally, at-will is just a legal default, someone could still negotiate a contract that required some different higher level of cause.
So as always, need to consult the specific state law and contract in question unless it's a clear violation of something federal like the Civil Rights Act.