where is this coming from 

An excuse used by shitty people to deny ownership or knowledge of their bad qualities. Can also be used to devalue and avoid change or responsibility over what the other person said to them. In either case, the speaker feigns surprise at these "new" and "unexpected" things they are being told.

Translates directly into "you've taken all my shit before, why the sudden backbone?" and/or "I've known this for awhile, but why bring it up now?"

Ellie: You lie to my face and behind my back, we only hang out when you need something, you ditch me the second you get a boyfriend and come running back the second he leaves you, and you give me your clothes that are too tight and then tell people you gave them to me because they were too big. You're a bad friend, and I'm so done with you.
Christie: Whoah, where is this coming from?

Dan: We've been friends for a long time and obviously you know I like you because I go above and beyond the call of duty of friendship, so now that you're single, maybe we can go on a date?
Jamie: A date? Like romantic? But you're my friend. Where is this coming from?

where is this coming from by WordsmithOwl August 6, 2016