When somebody suddenly becomes upset because of something another person did, then it is like a button was pushed. There is a certain type of action, or a certain implied meaning that blows a fuse in the person's mind. We need to find out what fuse it is before we can work on what it is connected to.
One might get upset from somebody not listening to what one says, from being invalidated, from not getting something completed, from not being liked, from words other people use, from being reminded of something painful, or from many other things.
Generally it is always something hidden that causes the upset. It is triggered by something that is out in the open, but there is a counterpart that is not. For example, one might get upset by something a person says that one perceives as insensitive. More fundamentally that is because one expects the other person to know the content of one's mind. The person "should" have known how you feel and think and acted accordingly. But the other person didn't know, and probably you yourself didn't know that was what was going on.
Fundamentally, anybody is creating their own upset, it is something they are cause over. However, at first shot it might work best to identify what it was that triggered it. Then later we can work on what it has to do with the person herself, and why and how she would create the reaction of being upset.
Generally speaking you can simply ask the person what it is she is upset about and she can answer it. Or you dig a little bit and go through the trigger incident and sort out what exactly happened.
Another approach is to have a canned list of likely ways one might get to be upset. That requires less skill of the facilitator, and is also a nice thing to have around as a last resort. There are many ways of making such a list. It is just a list of many different ways of getting upset. Invalidations, unacknowledged communication, mis-understandings, etc.
A simple but effective way of handling upsets with other people is with a little technique called the Four Magic Questions:
1. "What did ___ do that wasn't alright?"
2. "What did ___ fail to do?"
3. "As far as ___ is concerned, what did you do that wasn't alright?"
4. "As far as ___ is concerned, what did you fail to do?"
These questions are general enough to cover most reasons for upsets, but the client is likely to provide the exact keys producing the upset.
If your client has an upset with you, you can put in "I", otherwise the name of the person the client is upset with.
Another type of questions along similar lines would be:
"What should I have known?"
"What should ___ have known?"
"What should you have known?"
"What should have been known?"
Which ones you use depends on the situation. But basically the idea is that the upset is there because somebody didn't know what the other person expected and therefore acted differently.