What is the significance of 'Rosh ha'Pe'or' to which Balak now took Bil'am?
Rashi: Balak, who was a great sorcerer,
foresaw that Yisrael would be stricken with a calamity on that spot,
and he thought that perhaps this pertained to Bil'am's curse taking effect.
As is always the case with astrologers (and sorcerers), who foresee events without fully comprehending what they are seeing. What he saw of course, was the sin with the Moabite women which follows the current episode.