Clearly, the possibilities are numerous. Even though you're not likely to ever know which ones truly account for a particular person's behavior, acknowledging the possibilities can (if you're willing) lead you to explore alternative approaches to working with that person. And those approaches may turn things around.
The members of this particular IT group decided that it was Ms. C's need to have her views acknowledged that caused her bad attitude. They decided that in future interactions with her, they'd modify their behavior based on that assumption. And so they did. Over the next few months, they let her tout her ideas and voice her grievances. They listened without interrupting. (Yes, it was difficult!) They asked questions and empathized. They solicited her opinions. They tried to find positive points in her venting, and in so doing, they actually found several.
The change wasn't immediate, but over time, her aggressiveness diminished, and then, to their amazement, vanished. Furthermore, Ms. Easy-to-Work-With (Ms. E as they came to think of her) started becoming attentive to the pressures they were facing and, for the first time, empathized with their challenges.
The result: The stressful quality of their relationship disappeared, and a supportive relationship developed. Where once they were adversaries, now they gradually became allies. One of the unanticipated benefits: Ms. E began to alert IT to problems in her department that enabled them to head off potential snags in the work they were doing for her department.
Was a failure to have her views acknowledged at the root of Ms. E's stress-inducing behavior? Who knows? The important point is that if the IT group members hadn't changed their behavior, it's unlikely she would have changed hers. But they did, and the outcome was positive for all concerned.
Try it yourself with those who seem driven to demolish your sanity. Consider the possibilities that could account for their behavior. Modify your approach based on what you come up with. And maybe, just maybe, the troublesome aspects of your relationship will give way to mutual respect and collaboration.