I'm guessing the cause may be a simple 'equal to" rather than "greater or equal to" statement in the coding for checking if the population is at its max before the villagers reproduce. All I know for sure is this:
I had a village that I'ld founded from pulling zombie villagers out of my zombie spawner and curing them. They reached maximum population for the number of doors I had. Then another zombie villager showed up at night, I cured him. Next thing I knew, the villagers were reproducing children nonstop.
I also encountered this issue when a villager had maximum population, then I put an opaque block over a doorway, making it no longer a 'valid' door. The villagers immediately started mating. It seems that, when a village population is at its maximum, and either another villager is artificially added, or a door is made invalid, the population is no longer at max (it is over max) and the villagers start reproducing. The result is a lagging, unmanageable horde of villagers.
The attached picture shows where i've demolished all but one valid door. as you can see, there are still mating hearts being produced.
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Related to MC-3657.
Possibly duplicate.
I've had the exact same scenario happen. A village of rescued zombie villagers. I placed some blocks over the doors and noticed more babies than normal 5-6, by the end of the day: 100's of villagers and so much lag. I'm new to posting let me know if I leave out anything or am posting this in the wrong spot.
minecraft 1.4.7 / MCP v7.26 /FML v4.7.4.520 / Minecraft Forge 6.6.0.497 20 mods loaded, 20 mods active Voltz server
BTW: no new golums showed up, just the one original was there.
Hope this helps someone, thanks
Some more details, such as System, Java version, world details, and wether or not your Minecraft is modded or not are greatly appreciated.