The ATC convention, worldwide I believe, is that once the crash alarm is pressed, the surface of the aerodrome is "owned" by the emergency services who have the right to proceed as and where they like, and ATC can no longer guarantee the sanctity of their domain. Thus, the aerodrome is closed to all operations, except subsequent emergencies.
Once any emergency site is secured, control is formally handed back to ATC and the a/d reopened.
G'day
