Zombies picking up eggs and filling up the mob cap. 

I know that you made it so that zombies can't pick up glow ink sacs.

The thing is when you havent lighted up the caves around where you are building the will be jockies spawning. After the baby zombie sufficates in a wall the chicken will still be there and laying eggs. A lot of zombies will spawn and some of them will pick up the egg item and can't despawn. I would say its the same thing with glow ink sacs and eggs.