How to reproduce:
1. Put a boat in some water
2. Put a mob in the boat*
3. Get in the boat yourself
4. Rotate left or right (at least 180° or 360°)
--> The mobs head will try to stay in its rotation, hence the effect.

More information in the screenshot below.

*Not all mobs will work (Horses, Shulkers, etc.), so I used a cow.