You do not need to think of a trauma or anything like that. In fact, that is not recommended nor helpful. That is the beauty of doing these stretches/exercises—it doesn’t require the mind at all, you just do them, and the release happens through the body itself.
However, as you are stretching, thoughts/memories of past experiences or emotions may come up—that is fine and normal and this sometimes happens as they are being released. Don’t fight these but just take notice of them and they may have relevance or deeper insight for you.