First, this is not a duplicate of MC-16311, see below.

I posit at least one of these possibilities:
1. Zombie spawn conditions have changed (light level?)
2. Zombie behavior (detection skills?) has changed
3. Zombie sieges event activity has changed (happens all the time or never ends?)
4. The change between 13w22a and 13w23 made my lit caverns count as unlit, and would perhaps require removing and replacing all of my torches to count as properly lit.

Unusual number of zombies spawning detecting now in 13w23a and b.

Underground cave-system home (much like Etho's LP on the Ethoslab Youtube channel) had manageable numbers of zombies coming during 13w22a, but now is virtually swarmed with them.

This is not due to social zombies as I have observed during the course of other 1.6 snapshots, these waves of zombies appear with brief pauses, not directly from damage on an existing zombie (usually enough time to make another sword or eat a porkchop, and maybe regenerate some health).

Minutes after loading up my world in 13w23a, my wife and I battled hard, then I burned through 2 stacks of torches lighting additional areas in the cave system while she kept the horde at bay, but to no avail... it's like these zombies can sense us or our villagers from 50-100 blocks away through walls.

I do have a few cured villagers, but none of my doors have blocks exposed to sky on either side in a direct line (cave home). Without 'valid' doors, this can't be a village, and therefore can't be subject to sieges, right? Even if it were a siege, sieges end at some point, don't they?