I don't know how you could have stayed with the bar. You didn't dump it intentionally or walk out from under it. It popped off your back. As a ref, I wouldn't even warn someone for that. If you compete long enough, things will go sideways at some point. They have for me.
I agree with whoever said pull the bar down tight into your back as you ascend. I don't know that grip width would cause the issue. If you are fortunate enough to have the shoulder flexibility to keep your grip in tight, I think it would assist in keeping your back tight. The wider I go with my grip, the looser my back and less bar control I have.
That's a disappointing fail after locking it out.