Zorn said Friday the incident between his starters was "just a lot of nudging and posturing" and did not involve pushing, shoving or fisticuffs. He said he did not know what caused the dispute.
WRC-TV reported that Portis and Sellers had a heated argument, with Portis venting frustration over Sellers' blocking.
Neither Portis nor Sellers were made available to reporters after practice.
The week has been full of distracting news for the Redskins. Sherman Lewis was hired as an offensive consultant Tuesday, and defensive coordinator Greg Blache began an unexplained media boycott Thursday.