This is just a reminder to keep an ear out for the whistle and if you hear someone yell out STOP. You should also do the same so that the message is passed down the line. All archers even those at full draw should ease their strings forward, lower their bow and remove the nocked arrow.
Worst case in this incident could have been an injured or killed person. Best case in a competition is that any arrows fired after the whistle result in the highest scoring arrow being removed from your total. You could also possibly be disqualified for a safety breach.
Safety is everyone's responsibility, look out for it, look out for your fellow archers and listen out for the whistle. Please.