Upon noticing a mob it normally kills or a player, Withers will become "stuck" if there are blocks where its flying height is. However, it will be able to break through if it becomes damaged.

Steps to Reproduce:
1. Make a new Creative world
2. Create the structure show in the pictures and spawn pigs in it.
3. Spawn a Wither far enough from the structure so that it will not get destroyed.
4. Observe how the Wither continually pushes its self in to the blocks.

There are many different ways to trigger this, I just decided to do it this way.