Recent sightings of the Orcs portray them as far more skinny then they had been previously, an obvious sign that they are struggling to secure proper sources of food.
The Orcs probably held off on attacking the humans until hunger got the better of them, and they came looting the humans even though they remembered the warnings that their superior's had told them before we slaughtered them all. Since the crop fields were almost totally undefended, the Orcs also struck there.