There could be numerous causes for this,

a) There was some natural calamity like tornado or storm that caused it.

b) - There was no food left for those birds in that area.

c) - Due to human intervention environment in that area, survival became unsuitable for them.

d) The population of their predator might have grown huge which ate all birds.

But taking into consideration, it was a mass extinction which occurs mostly due to environmental factors.