Lydia has some kind of power over the birds
There is a theory that the birds are a metaphor for Lydia's hostility towards Melanie, but what if Lydia had some kind of psychic ability towards birds of which she is not aware of and has no power over, involuntarily causing the birds to swarm and attack Melanie and the rest of the town when her anxiety about Mitch leaving her alone. It would explain why the gull attacked Melanie first (maybe Linda spotted her leaving) and also why the birds seem attracted to the place where Melanie is at the time (especially at Cathy's party, where the birds attack just after Lydia spots Mitch and Melanie talking on the hill and looks at them like he has betrayed her)
The lovebirds are the masterminds.
I've always had a theory about what caused the birds to start acting so differently: the lovebirds. Somehow they orchestrated everything - this would explain why the attack at the end seems to be concentrated mainly, if not solely, on the Brenner house.
Global warming was causing the birds to attack
Thankfully the birds did not enter the exploding phase
that global warming causes.