Why turn your back to Cass ult... (Syndra pls)

If you can just turn her around and save everyone?

Just learned today that Syndra E turns Cass around, and I'm not sure if it was luck, delays, or animations, but after I'd hit R, Syndra would land an E and turn me around before the ult actually cast. Cool trick. Completely denied me 2 good ults that game, and another 1v1 ult that would have guaranteed me a kill.