In the first posuk (Numbers 22:2) it says:
וירא בלק בן ציפור (And Balak the son of Tzippor Saw)
no mention of his Royal status, later on in the parsha (22:4) it says:
"ובלק בן ציפור מלך למואב (Balak the son of Tzippor was a King of Maov)
What happened that only later the torah tells us that he was king?
The Ohr Hachayim asks: a) Why does it say Balak saw what Israel did... The entire Moav was scared, it should say Moav saw. b) Midyan had plenty to be scared of, too, but the pasuk doesn't mention it.
He answers that Balak was really part of the chiefs of Midyan. When the Torah says Balak saw, its like saying Midyan was worried about the situation. When Moav grew fearful, they then recruited Balak to be their king. (see seforno that he was skilled at war, but the O"H doesn't say why)
The Brisker Rov answers at first he was not King, it was only after he said theses Jews bring all the suffering in the world then and only then did they make him a King. Like the Gemara says in Gittin (56b): "המיצר לישראל נעשה ראש"
Ramban (Numbers 22:4) asks this question, and says he doesn't know the answer:
ולא ידעתי טעם למה שאמר הכתוב ובלק בן צפור מלך למואב בעת ההיא - כי הראוי הוא שיאמר תחלה, וירא בלק בן צפור מלך מואב את כל אשר עשה ישראל לאמורי
Rabbenu Bahya (22:2) suggests that the Torah omits the title "king" from Balak when describing how he saw what what Israel did to the Emorites, since he became so scared, as a result, that he no longer felt like a king. When it later says that he "was king", it means that he was previously like a king at the time of the Emorite defeat.
