Macedonian nationalists were encouraged at various times by serbs, austrians, and russians, mostly to discourage various other nations from trying to integrate them.
That said it's not fair to call it a foreign movement- macedonians did come to consider themselves a nation internally. The father of macedonian nationality was Georgi Pulevski, who did amateur research and writing and elaborated on (IIRC) the influences of albanian, turkish, and greek culture on the "macedonian" people and language- he considered himself an ethnic bulgarian and a "serb patriot" but nevertheless argued for a unique "slavjano-makedonski*" identity. There were macedonian nationalists going to Russia in the 19th century to seek independence from the turks (as "slavjano-makedonski", not as bulgarians or serbs), although I wouldn't say macedonian identity was fully-formed until Serbia and then Bulgaria occupied it briefly. The Kingdom of SC and C had tried to enforce Serbo-croatian on Macedonia, and then during WW2 the bulgarians tried to enforce bulgarian- as it was, being forced into either camp spurred macedonians into the hands of the third way and they largely supported the communists and got their own republic in return.
*the macedonian nationalist movement in real life never really claimed to be the sole inheritors of ancient Macedonia (though I'm sure there are some modern, uneducated crazies who would), which makes the greeks look a bit crude when you see how angry they get about it. The macedonians merely claim that the ancient hellenic(-influenced) culture was one of several that the slavs who now live there were influenced by. The did not in any way deny that macedonians were slavs akin to bulgarians and serbs.