An outbreak of avian flu or any other contagious disease can happen quite easily. Some of these have a period to survive of just hours to days, other may last much longer, up to even months.
An infectious and specifically zoonotic disease is not just harming poultry. It may also seriously impact human health. After all, poultry and poultry products end up in the food chain and thus may seriously impact the health status of food products from this origin.
In the event of a highly contagious disease, there is no alternative but culling flocks and taking measures to get the disease under control again. This usually happens.
Such a measure, like in The Netherlands, is a ban on transporting poultry and poultry products, initiated by the government, in the area where a disease outbreak had happened.