I have a heavily modified village. For some reason, the villagers won't breed. But they are spawning iron golems just as fast as they can! I discovered that I have an abandoned mineshaft directly below the village (the subtitle said "minecart rolls" while I was standing on the beds, and I have placed no minecart tracks anywhere near here.)

 
I think the villagers can see the zombies and stuff spawning under the ground somehow.