This is a mystery that is never fully solved in the story. What Nat and others assume at the beginning, before the bird attacks become too serious, is that the birds are acting in such strange ways because of the harshness and severity of the winter, which is making them come inland and attack humans in scattered, isolated incidents, because of lack of food. However, when it becomes clear that something much more sinister is going on, and that the birds are turning themselves into killing machines intent on the destruction of the human species, this reason becomes null and void. The only suggestion that is made in the text is in the last page of the story, where Nat talks about the birds and their capacity to remember:
Nat listened to the tearing sound of splintering wood, and wondered how many million years of memory were stored in those little brains, behind the stabbing beaks, the piercing eyes, now giving them this instinct to destroy mankind with all the deft precision of machines.
This suggests that there has been some sort of latent evil or hatred of man that exists deep within birds and has only now found its full expression. The way Nat talks of "millions of years" hints at a kind of primeval antagonism between the two species that now makes them act as they do. They are hardwired to execute humans, but only now has this become apparent. In a sense, the story becomes more terrifying with this lack of certainty about the reason behind the behaviour of the birds.