When a player walks through a block which is transparent or translucent in some way, the third-person camera forces the viewport to not exceed the internal boundary of the object, blocking the view until the player leaves the block which encases their head. This visibly affects all tall plants, the Nether portal block, and presumably anything else which is a non-opaque block but doesn't fully block the player's motion.

This would make sense if the blocks were physically solid, but you can easily walk through sugar or grass...

To reproduce:
1. In a loaded Minecraft world, switch view to third-person.
2. Walk into a certain block (such as tall grass), and try to move the player's head into the block. Notice what happens to the camera.
3. Walk out of the block. Notice how the camera is now at the maximum viewing position again.

This can be repeated for any of the other blocks as are described above.