In 1.8, village boundaries for villagers, or the area in which they can walk around without leaving the village and walking back, have been extended by about 10 or so blocks. However, iron golems don't seem to follow this new boundary, and are still stuck in the smaller area that the villagers had stayed in in 1.7. This causes many problems, such as iron golems not attacking zombies when the villagers are too far away from the village, since iron golems never attack mobs beyond the village boundaries.