often times nobles start to develop when one family starts getting really powerful. If one guy has dozens of brothers, cousins, sons and nepewes he can call on to back him up he has a lot of force to throw around.

a related aspect is wealth a powerful family has a lot of it, this wealthy can be used to get other lesser families to work with you.

intermarriage then expands the power basis as other strong but not quite as strong families want to get in on the act.