What exactly do you mean by a serious incident though? There certainly have been fatalities in groups of cyclists, out for club runs and similar, which I'd say counts as pretty serious, and there have been fatalities on large organised rides, I know of one on the Dun Run (which admittedly is fairly lightly organised), and I'd imagine there have been others.
It would be almost impossible to legislate against group rides, after all what exactly would that cover? Even on commutes, I can find myself surrounded by a dozen other cyclists, by no sane definition would that be a group ride, but it could easily fall afoul of a legal definition designed to stop people going for a ride with a group of people (like a FNRttC).
Racing is slightly different, and it would be difficult to use any legislation against group riding to stop a time trial. Time trials of course actually have their origin in the potential risk of the authorities trying to stop racing on public roads, hence the historical wearing of all black, and the code names given to race routes.