'Wild 'N Out' Video Reveals Why Azealia Banks Went Nuclear

Now we know what incited Azealia Banks into a fit of racial rage last month ... DC Young Fly's joke on "Wild 'N Out" that Azealia was a poor substitute for Cardi B.

TMZ has obtained a clip from the show, during a game called "So Petty." DC cracked that Azealia was booked because Nick Cannon and co. couldn't land Cardi.

Azealia didn't find it funny at all, calling the punchline "anti-black." The intimation ... that Cardi B was a light-skinned woman and therefore more beautiful than the darker-skinned Azealia.

So, now we know why Azealia went HAM on social media last month, saying, "Some little scrawny black Atlanta twink was prepared for me. They planned this - hit me on short notice, told me I didn’t have to participate in the coon ass freestyle battles, (not my style) and I was to sing my song and go home."

Nick was clearly annoyed by Azealia's comment, saying essentially she opened herself up to attack by resorting to racial insults.