The Zohar (III parshas Shelach p. 158a) says that the spies spoke badly about the land of Israel because they felt they wouldn't remain in a position of leadership once the Jews entered the land. To maintain their power, they tried to convince the people to abandon the plan to conquer the land. This Zohar is brought by many sources, including Mesillas Yesharim (Chapter 11) and the Shelah (parshas Shelach Torah Ohr 3).
The Be'er Yosef suggests that they weren't making an assumption. They saw a vision that they wouldn't be the leaders once the Jews entered the land (side point: it ended up coming true, as they died, and thus weren't leaders when the Jews entered the land). However, the Zohar itself, and the sources above that quote it, don't mention anything about prophecy. It sounds more like paranoia, unwarranted fears.
Is this the Be'er Yosef's insight? Or is there an earlier source for such an interpretation. Or perhaps, this is the simple reading of the Zohar.