Memories are part of our information processing equipment and so are kind of like thoughts or feelings; they are evoked when something "related" evokes them. It could just be this person reminds you of your father in some unknown way (like dream images and your unconscious) or at the moment it is "easier" for you to think about him than your father, etc. It is interesting that he has come up stronger than your father; it's interesting (to me :-) that I think about and miss my father-in-law more than my own father, although I had a good, loving relationship with my father, too.