The most similar groups tend to have the harshest
rivalries.
When you look at subcultures, peer groups, personal and professional associations, there tends to be rifts that only those in the group are aware of, or even care about. Usually, it is a subtle difference that is elevated in importance. (Those in the group don't think the difference is subtle at all).
EX: Ski vs. Snowboarding, MAC vs. PC, Giants vs. Dodgers, Republican vs. Democrat, Protestant vs. Catholic, etc. Someone who does not derive their identity from either side see a more general picture: snow sports, computers, baseball fans, politicians, christians. These are simple examples, but the concept can be applied to many subgroups. Almost any way you can describe a group, there will be a subgroup split.
It is a strange group dynamic to focus on differences and forget the similarities. I guess it's kind of a group identity crisis. People want to distinguish themselves from what they consider they are not.
Are splinter groups the natural result of too much success? When a shared interest is held by a small number of people, they tend to overlook the differences. As soon as there are many, the group tends to split. Both groups think that they are better than the other in some way, either in methodology or general righteousness. They start to deny that they have anything in common. This is one way a rivalry is born.
Another way a rivalry starts is two groups arriving at a similar situation, location, functional role, status, etc. (but that's a different node) Still, there is a denial of similarities that is embedded in the rivalry.
What surprises me the most (maybe it shouldn't), is that groups often hold on to their rivalries at the expense of there own best interests. They undermine the common interest, rather than simply validate the differences and get on with their lives. Like I said before, everyone outside the group either doesn't see the subtle difference or doesn't care.