What is the problem with an influx of wild beasts? Surely Hashem can save Yisrael from them, just like He would save them from the Cana'anim?
Rashi: Because Yisrael were less numerous than the nations that they were destroying. Consequently, until they will increase, wild animals will inhabit the sparsely inhabited towns. 1
Targum Yonasan: The wild animals will come in droves to devour the corpses of the enemy that Yisrael killed and attack you. 2
Hadar Zekenim (in Pasuk 20): The Pasuk is referring to the Yeitzer ha'Ra - When you see that the land becomes desolate of Cana'anim, your Yeitzer ha'Ra 3 will increase and incite you to sin.
Rashi: And Yisrael will not deserve to be saved from the Chayos - bearing in mind that Yisrael conquered the land due to the sins of the Cana'anim and on the merit of the Avos, and not due to their own merits (See Eikev Devarim, 9:5) PF. See also Ba'al ha'Turim.
Perhaps Targum Yonasan concurs with Rashi in answer #1.
