I can think of five potential explanations but there are many more. One possibility is that you misinterpreted the situation. Perhaps your therapist had something in her eye that caused the tear. It could have been a piece of dust, lint or an allergic reaction.
If she was in fact crying, it may have been an expression of sympathy or empathy. You did not describe the nature of your story so I cannot know for certain if that explanation is a possibility.
A third reason may be that something you said reminded her of a sad life experience. This is not an infrequent occurrence. People tend to be emotional about stories or situations that are similar to those that they have personally experienced.
A fourth explanation is the possibility that your therapist was not feeling well emotionally. Lastly, she could have been experiencing physical pain that was becoming increasingly difficult to tolerate during the session.
As you can see, there are many possible explanations. If you want to know what happened, it is best to ask your therapist. Most therapists care very much about the well being of their clients. Your therapist, very likely, does care about you and your pain and suffering; both present and past. It is perfectly all right to ask your therapist why she or he was crying. Very likely your therapist was feeling both sympathy and empathy about what had happened to you. Don’t be afraid to be more open with your therapist. In existential therapy, for example, it is very desirable for an emotional bond to develop between the client and the therapist. You can read more about this in the work of Rollo May. If you have any more questions, please write back. Take care.