They act like that every year for 3 months. They guard their nests/newborns by attacking people, other birds, cats and dogs, thus letting any predator know that they have their nest nearby. If they had more natural predators it would be an animal bound to go extinct.
I cannot stand them, mainly because they bully other birds. I've seen them attack sparrows mid-air and kill them just for the sake of it. They get their asses kicked by swallows though, they are much faster and more agile.