I don't know.
Joshua and his tribe were compared to oxen and beasts, not falcons. An unlikely explanation brought by Israeli professor
Reuven (Ruvik) Rosenthal is that like a falcon who hunts down his prey, so did
Joshua conquer the land of
Canaan. But the professor admits it's amusing, yet unlikely. Perhaps it started with one jew named
Yehoshua Falk and it gained popularity, and there were famous rabbis carrying that name as well.