It can happen at anytime during the squat to be fair, it depends on where the weakness lies, or if there is no weakness where they loose tightness due to lack of motor control, probably the main reason for most in my opinion.
You are right that we need to see a video, my point was simply that without one neutral would make a better cue, here is a basic illustration of the different spinal positions so you can get where I am coming from.
As you can see from the diagram a 'neutral' position will still have a slight curvature of the lumbar spine. Maybe this is what you were getting at the whole time though. I am unsure.