When a zombie pigman, skeleton, or wither skeleton wanders into a full-grown nether wart bed in a nether fortress, it becomes immobilized. Because they sink into the soul sand and the bed is surrounded by blocks, they cannot jump high enough to get out. Furthermore, they see the nether wart as a block they have to jump on top of, and when it's full grown they cannot jump that high either so they can't even move around within the bed.

If you break all of the nether wart in a bed, they can move around on the bare soul sand but still can't jump out. If you break part of the nether wart, they wander around until they encounter a nether wart and try to jump on it. Occasionally when they do this they land on a nether brick block or stair, and from there they can walk away. That is the only way they can escape, but it seldom happens and can only happen if some of the nether wart is missing.

This causes skeletons and wither skeletons to become concentrated in the nether wart beds over time. They apparently don't despawn there, so eventually their mob caps are reached and they stop spawning in corridors. If there are enough nether wart beds, they fill the density cap and magma cubes and ghasts stop spawning as well. Except for the nether wart beds, the entire fortress becomes deserted. Or, you can easily pick off the mobs one by one since they can't chase you.

Steps to reproduce: (Easiest in Creative)

1. Find a nether fortress.
2. Look for nether wart beds. Note how many mobs, if any, are in them.
3. If the bed is full of nether wart, watch and note that the mobs do not move around.
4. Wait 20 minutes or so. Follow mobs as they wander if you like.
5. Revisit the nether wart beds. Note that they have more mobs in them now and that none have left.
   You can continue waiting and revisiting until the beds become full of mobs. At that point you should notice that few if any mobs are encountered in the corridors, and if there are several nether wart beds in the fortress you may notice that no magma cubes or ghasts are spawning in the area.