Couldn't this simply be a test of the reciprocity principle? People who feel like they've wasted someone else's time are probably more likely to make an effort to resolve the problem so as to be less of an inconvenience to the other person, or to meet the other person's expectations. If the mediator is chipper no matter what, then nothing is socially at stake for the negotiators, and it shows that the mediator is unattached to the outcome(which isn't or shouldn't be true). On the other hand, the perception of being a nuisance also comes with the social implication that one isn't pulling their own weight of effort, and most of us naturally want to be judged well by the group we are a part of.
So perhaps it's less about hostility per se, but the covert social communication made through the hostility.
So perhaps it's less about hostility per se, but the covert social communication made through the hostility.