It's highly dependent by the state in which you work. Github is in California, an "at-will" state [1], which basically means that you can get fired for any reason, including no reason at all. The only exceptions to this rule are if you were fired as a result of discrimination of a certain protected characteristic such as your age, gender, race, etc.

[1] http://www.business.ca.gov/StartaBusiness/AdministeringEmplo...