I finally found an article. I believe their fight is rooted in their producers scott storch and timblands big publicized rivalry right now.
Fergie to Nelly Furtado: "Say it to My Face"
The Fergie/Furtado feud flames on!
On "Impacto," Fergie's latest collaboration with Daddy Yankee, the self-proclaimed Duchess fillets the "Promiscuous" singer, saying, "I ain't promiscuous, the realest chick up in this," and threatens with an old fashioned "I'ma pull your wig back!"
This isn't Fergie's first shot at Nelly. In "Fergalicious", she also says that she's not promiscuous.
TMZ