My theory was, a long time ago, that Dragon had, while he was a marine, a job to protect a CD daughter and fell in love with her, They started a relationship. The CDs and Gorosei wanted to stop that, searched for them, they fleed. She gave birth to a son (Luffy) and shortly after they got caught. While Dragon and his baby could flee, without the Government noticing an existing baby, they caught Luffys Mother and imprisoned her.
Thats why Dragon went to the Revolutionaries, since he wants also to free his wife. And also explains why he is the most wanted man.