This is out of the owners manual. It sounds like what you were experiencing.
While driving in Sport Mode, if Tap Shift has not been activated, the transmission determines when the vehicle is being driven in a competitive manner. The transmission may remain in a gear longer than it would in the normal driving mode based on throttle input and vehicle lateral acceleration. If there is a rapid reduction in throttle from a heavy throttle position, the transmission will maintain the current gear up to near redline rpm. While braking, the transmission will automatically downshift to the next lower gear keeping engine speed above approximately 3000 rpm. If the vehicle is then driven for a short time at a steady speed, and without high cornering loads, the transmission will up-shift one gear at a time, until 6 (Sixth) gear. After shifting to 6 (Sixth) gear, or coming to a complete stop, the transmission will return to normal Sport Mode shifting.