Broken Zombies

I was mining when suddenly a zombie attacked me. I hit him with my sword and then he just stopped moving and stood still shaking his arms up and down (video attached). Also, I don't really know how this happened.

What I expected to happen:
A zombie attacking me, even after I had hit him.

What really happened:
The zombie stood still shaking his arms

Steps to reproduce:
1. Get a zombie to attack you
2. Hit him
3. Watch him