Airports have agreements with Local Authority Fire and Rescue (they have to under Civil Contingencies Act) and in that will be a PDA level (pre-determined attendance).
What this means is that they agree that if they have a certain type of emergency then the Local Authorities (Police, Fire and Ambulance) will send an agreed minimum number of appliances to ensure appropriate facilities are in place for the Worst Case Scenario in each category of incident because even if an airport has ARFF Cat10 they would not have the resources to fully deal with even a 737-300 (Cat 6 a/c) crashing on the runway.
For example this could be:
Aircraft Accident: 4 Police Officers, 1 Police Supervisor, 4 Ambulances, 2 Ambulance RRVs, 1 Ambulance Officer, 6 Fire Tenders, 1 Fire Command Vehicle
Full emergency standby (Pax): 4 Police Officers, 1 Police Supervisor, 4 Ambulances, 2 Ambulance RRVs, 1 Ambulance Officer, 6 Fire Tenders, 1 Fire Command Vehicle
Full Emergency standby (Cargo): 4 Police Officers, 1 Police Supervisor, 1 Ambulance, 1 Ambulance RRV, 3 Fire Tenders, 1 Fire Command Vehicle
Aircraft Ground Incident: 1 Police Officer, 2 Fire Tenders.
It's a precautionary thing and (certainly where I am) they are dispatched by an auto-dialler indicating the level of PDA required with the Police getting a follow up phonecall with initial details and receive full updates from the ARFF fire control as to the situation once they start making their way.
This will have been classed as an Aircraft Ground Incident (AGI), and possibly even an Aircraft Accident, and will therefore have received the appropriate PDA agreed for that. They may not be, and were not, needed but the Initial Action procedures have to be rigid in saying - it's an AGI so activate AGI PDA and if the ARFF commander then declares an aircraft accident, activate the AirAcc PDA (unless the ARFF Cdr says he does not need further local authority assistance)
Don't forget - ARFF categories are not about being able to fully deal with an incident but to be able to CONTROL a worst case scenario incident to increase the chances of survivability until Local Authority can provide backup. Where situation occurs that they are able to fully deal with to conclusion then local authority can be stood-down but until it is clear that the ARFF can deal with it themselves the Local Authority will always start making their way to the Rendezvous Point.