A phrase used in substitution for actual reasons. It's typically used in situations where a question is either too complex to give a sound logical answer and it's simpler to convince the other person that they should just give you the benefit of the doubt, or because the question is incredibly stupid and therefore not worth answering, or the answer that would have been given should be obvious to anyone with a brain. It can also be used to just make a statement about something.
Person 1: I fucking hate Carrot Top
Person 2: Why?
Person 1: Because reasons.
Person 1: Are you voting for Santorum
in the primaries?
Person 2: No, because reasons.
*kid falls off of bicycle*
Person 1: Haha, it's funny