Historically speaking, Orthodox Jews are the ones who disagreed with the Reform movement.
Reform Judaism is only a few centuries old, originating as part of the Haskalah, the Jewish adoption of Enlightenment ideas. Other movements are newer, either breaking away from Reform like the Conservative movement did or being modeled on it.
When the Reform movement first began adopting changes to traditional Judaism, "Orthodox" became the catch all term for those who rejected said changes.
Which is not to say that Orthodox Jews are living as they would have if there were never a Reform movement; in many cases, Orthodox Rabbis and communities made decisions specifically to counter Reform rulings.
The difficulty is that, ultimately, there is no Orthodox movement. Reform, Conservative, and Reconstructionist are all distinct movements with official rabbinical counsels who can make rulings about what the movement officially believes (though of course that doesn't mean every Jew who is part of those movements holds those beliefs) there is no comparable Orthodox counsel. "Chief Rabbi," of Israel or elsewhere, is a government position. In Israel, the Chief Rabbi can make rulings such as who counts as a Jew for the purposes of the Law of Return. In other countries, the Chief Rabbi tends to act as a representative of the local Jewish community to the government.
But while they are (usually) highly respected and influential Rabbis, that doesn't mean they have the authority to make official rulings for all Orthodox Jews, or even all Orthodox Jews in their country.
Now, there are Orthodox movements based on the teachings of specific Rabbis and their heirs, so those movements will have official beliefs, but then you get something like Modern Orthodoxy, which is really more of a philosophy than a movement, and the borders of who is and is not Modern Orthodox can get really fuzzy.
And, crucially, all of this only applies to Ashkenazi Jews.
Reform was a European movement. All of the political implications of Reform and later movements (and yeah, a lot of this is political) are within Ashkenazi Judaism.
Other minhagim aren't Orthodox, because Orthodox just means "not Reform, Conservative, ect," and they don't have a Reform movement to be "not".
This doesn't mean that other minhagim are more or less traditional that Ashkenazi Judiasm, just that the terms don't apply.
I hope that this cleared some things up!
(I assume that it did not.)
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