The horse that spooked was a seasoned champion and they still have no idea what caused him to panic like that - they figure he was somehow stung, or his tail had gotten hung up in the tracing and he couldn't get it loose. I believe they have retired him since this happened.
It's easy to say "do this, do that". When a horse is that erratic and you have no idea where he's going, opening the gates is the WORST thing you can do. You can try and believe it would be easy to evacuate, but the pattern of the bolt was all over the place and staying in the center WAS the best solution. Most horses don't bolt into the center of a large group but with blinders on, it's understandable how it can happen.
I absolutely ADORE the announcer in this video - she is a picture perfect image of cool as a cucumber and being able to effectively manage things without even being in the ring.