They had to physically stop because they bunched up from going too fast.
What normally happens on a restart is that the teams communicate when the SC lights are out. Which is generally at the last SC line. However, on a grid restart the SC lights go out at the end of S1 and it becomes the leader than sets the pace. In a formation lap they often do speed up and slow down - especially towards the end, where the leader will bunch the pack to avoid having to sit on the grid while waiting for everyone else. But we don't have these incidents on formation laps because it's a normal thing that happens at every race, so they expect it. A red flag and grid restart is not a normal thing.
The back of the grid was coming in too hot given the SC lights were out. It wasn't communicated to them in a regular fashion.
And of course David Coulthard, the Red Bull ambassador that is still on their books, thinks Lewis was wrong. He still thinks the best every qualifying lap was a Max Verstappen lap where he ended in a wall. We might as well ask Mr Spice what he thinks of Lewis. I am sure it will be a well balanced opinion.