They were so well mannered when they were young.
However, then they left the nest and joined roving youth gangs, grabbing what they could and harassing adults. We're not talking about your teenagers, but rather the surprising behavior of juvenile ravens.
Young ravens form gangs not to cause trouble, but to increase their chances of having a steady food supply through winter. When a hungry, young raven spots a bonanza of food on the forest floor, like say a big, dead moose, it won't dive down and start feasting. It flies away first to communicate its discovery with up to fifty other raven youth. This raven youth gang will then arrive together the next morning, and share the carcass.
Such sharing resembles selflessness and altruism, difficult concepts to reconcile with certain, old-fashioned notions of evolution. If evolution is survival of the fittest, why should a young raven share a winter's worth of food with its unrelated rivals?
Actually, this sharing helps a raven get more food, not less. In winter, big carcasses are rare and often widely scattered. An alliance of ravens can search far more territory than a single raven ever could, so the carcasses will be found more regularly. Moreover, while young ravens roam far and wide, adult ravens pair up to defend well-defined territories. Chances are, that delicious moose carcass lies inside one of these territories, and it is defended by the adult pair. While one or two juveniles wouldn't stand a chance against two adult defenders, a menacing youth gang can easily muscle their way in on the feast.