The girl who shoved "Teen Mom 2" star Jenelle Evans before that fight erupted ... is dropping a dime -- BIG TIME -- claiming Jenelle was actually in on a plot to make money off the brawl.
Brittany Maggard claims Jenelle only got up the guts to fight Britany Truett -- the eventual ass-beat victim -- because Maggard, Maggard's sister, and their friend Nick had her back. Maggard says Nick had the video camera.
After the fight, Maggard claims she and Nick came up with the idea to sell the video -- and Jenelle not only thought it was a "good idea" ... but SHE called "someone she knows at a [photo] agency to purchase the video."
According to Maggard, they sold the vid for $5,000 -- and Jenelle wanted a bigger cut because, as she said, "You wouldn't get any money if I wasn't famous!"
Jenelle's attorney says he doesn't believe Maggard's accusations.