There is no real difference between pack or herd mentality other than pack has a more violent meaning than herd. It's also like some prefer to be called a group or club and not a gang or mob.
Semantics (playing with the meaning of words) is something I enjoy. I was at a place not long ago that had a saying on the wall that was aimed at being an inspirational line. It said “obstacles are just challenges on your path”. I addressed the person at the desk “so this mean speed bumps and traffic lights are a challenge and not an obstacle”. I found it even funnier when she took me seriously.
So I was playing the semantics game with the pack mentality vs herd mentality.
Group talk is often used to try and give more power to what is said. It can have hilarious outcomes when questioned.
