Maybe clutch got stuck at high rpms? It use to happen to me a few times when I would shift around 7k rpms. Then I removed the assist spring on the clutch and problem solved.
2012 Mustang 5.0
Steeda CAI, Boss Manifold, 3.73 Gears, E85, NX Wet 150 Shot, AED Tuned, LT Headers, Aluminum Driveshaft, Mcleod Twin Disc Clutch, KB Bap, N2MB wot box w/ window switch.