Our Theory: "Suddenly" might be the wrong word. It's basically implied that the Engineers have been angry with us for some time, they being the "parents" who once created us. Scott has said there were sequences in the film that were going to explain why we'd angered the "gods" known as the Engineers. In one instance, it was going to be posited that Jesus was an emissary of the Engineers, sent to see how we were doing. And what did we do? We crucified him. So the Engineers have been upset with us for some time. But something happened while they were creating the aliens to force the Engineers into hibernation. This, also, falls under the category of "Things probably better explained in a potential sequel," which is disappointing for Prometheus, but there you have it.