Iron Golems Swing their arms rapidly when in a too small space

If an Iron Golem is in a 3 block high gap and an Enderman is as well, the Iron Golem will swing it's arms rapidly at the Enderman.

What I expected to happen was...:

The Iron Golem would have a bit of an interval between hits.

What actually happened was...:

It swung it's arms rapidly at the mob.

Steps to Reproduce:

1. Make the structure in the screenshot.

2. Spawn a Golem under it and an enderman.

3. Notice that is swings it's arms rapidly.