I can think of a clear motive for the mortal Primalists.
The Titans have always treated Azeroth as nothing more than another potential Titan, to be incubated and protected until she’s ready to be born. Any life that exists on her surface is nothing more than a by-product. In fact, such lifeforms are so insignificant in the eyes of the Titans that there are multiple countermeasures put in place to wipe everything out in order to prevent the corruption of Azeroth.
Even lifeforms like the Aspects are nothing more than custodians put in place to protect Azeroth until she’s ready to awaken and join the Pantheon.
I can understand why some people would react to the discovery of this grand plan and their irrelevance in the greater scheme of things with irrational anger. Azeroth is more than just a nascent Titan, it’s home to countless living things and it’s wrong of the Titans to not value their lives. Therefore, it’s only right for the inhabitants of Azeroth to fight against the Pantheon and strive to rid their world of Titanic machinations.
It’s not a very wise course of action, nor is it canon, but I can understand some inhabitants of Azeroth not being very happy with the Titans or their servants.