For us at least, it just happened naturally. I often talked to myself and felt as though there was definitely more than one of us, until one day our friend told us that we might be plural and we realized that we could really relate with the plural experience.
So, how did you figured it out?
We figured it out when we had words to put to experiences we had when the body was small.
We have been plural since then (when the body was small), most likely not from trauma only or at all, though some of us have been created due to trauma since then. The members have changed over the years though.