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.