Colleagues exaggerate problems in department
I don't know whether it would work in your situation, but I have often found that disagreeing in a questioning manner helps defuse more than disagreeing directly when dealing with this sort of 'build up' because explicitly disagreeing makes a person defensive and more rigid in their position. For example, instead of saying "Prof X wasn't being rude", you could try something like "yes, Prof X did seem unusually focussed this morning, must have had something on his/her mind".

But it sounds like nobody knows each other very well. Such problems tend to disappear when there is more interaction because people are more likely to attribute a bad interaction to 'must be having a bad day' rather than 'must be a bad person' when they know the person. Would it be possible to encourage more interaction within your department? For example, you could suggest a work-in-progress monthly session where PhD students and postdocs once a year or so talk about their work. If you have 4 people from different groups presenting, then more senior people will turn up as one of 'their' students/researchers will be presenting.