What someone says when they have done something for the purpose of maliciously disrupting another person(s) motive, intentions, life, job, etc, and the victim asks that someone why they did it.
NEWS AGENCY: We have hired a new science editor.
Bob: Welcome to the team, Freeman. We need you to start by editing this paper on climate change. Exxon is one of our top sponsors and they have told you to remove the bit on 80% efficiency solar panels and make it look like oil does not harm the environment.
3 days later:
Bob: FREEMAN! You included the bit on 80% efficiency solar panels, you made the bit on oil seem even more against the environment, and then you proved that Exxon bribes us! WHY?!
Freeman: Why? FUCK YOU, that's why.