Before you watch the video, I'd like to point out a few things.
I have tested the following.
- Different Pet
- Assist / Passive
- Swapped Talents around
From what I understand, Growl doesn't auto cast because this is not a "threat fight" he is meant to keep attacking the pet unless the pet dies.
Why do I keep pulling agro?
https://www.youtube.com/watch?v=F6-IbppYq0s