What kind of feelings might motivate this behavior?

I have a team of people working for me on a long term contract. For the most part, the team has been tight, and worked well together. However, a few months ago, one of the women on the team started verbally bullying one of the men.

It didn’t happen often, but slowly it grew. She seemed to take a hostile reaction to any proposal he made, even the good ones, and she seemed to start having a difficult time hearing what he was saying.

I’ve spoken to both of them, but they both deny that anything is out of the ordinary. He says it’s not a big deal. She says there’s no reason to change her behavior, because she’s just being herself.

Everyone else notices the friction, but it’s kind of like the elephant in the closet that no one sees. Do they just genuinely disagree? Then why the personal slurs? Is that just her way of arguing, and the slurs are nothing personal? For a long time, he didn’t seem rise to her bait, but lately, he’s started getting testy in response, something he rarely seems to do.

Could this behavior may be motivated by some kind of unconscious feelings? Do you have any ideas to help me explain this? If I have a good understanding of what feelings could be driving the behavior, and where those feelings might come from, maybe I can find a better solution to the problem.