[They] "were being used to grind FIFA Ultimate Team...with the goal of selling accounts loaded up with in-game currency on the black market."
...
Ultimate Team is the perfect game for this kind of operation, given how it's structured. You can spend real-world money on loot boxes in the hope of obtaining high-value cards, but the odds of getting one of the best players is soul-destroyingly slim. Or, you can play the game for months on end in a bid to save up enough of the in-game currency to splash out on the auction house. Or, you can buy FUT coins on the black market