It feels like you are asking this question because it pertains specifically to you. Since I don't know all the details about this young man and his family, I will try to answer in general terms.
Emerging adults want the freedom to explore and find out who they are. Perhaps this young man felt that his family was holding him back or pressuring him to make decisions that he did not want to make or was not ready for. He needed total independence to find his way in life.
Young people want to change things up. They want to try new experiences. If they make a mistake, they are young enough to change direction and recover from it.
Of course, this young man may have cut off contact for personal reasons having nothing to do with his life-stage. Perhaps there were simmering tensions in the family, and your son finally felt confident enough to make a break. He may have done this because he has not yet mature enough to handle his emotions in a better way. With a little time on his own and some personal growth, he may resume contact.
I'm not an advice columnist, but, just as a mother, I'll say that sometimes you need to "give kids their space." Respect his desire to be on his own, but let him know that you love him and you will be there when he wants to resume contact.