Camron Rust wroteThe only thing I saw that would support a flagrant is that he didn't just get run into but he chucked him with his arms....adding unnecessary contact.
That's exactly what they called the flagrant for...plus they probably erred on the side of upgrading to a flagrant foul because they didn't want the Thunder, who were getting drummed at that point, to start taking out their frustrations on the opposing team and wanted to nip that in the bud.
But the contact in of itself wasn't a flagrant foul IMO.