Other mistake: When Buck tells about his fight with Rudy, he swings while saying "Back, forth" in front of the others. They are looking the wrong way. He is also swinging on nothing at all.
Other mistake: As Sid gets Scrat stuck on his foot in the beginning of the movie, the sky and mountains shouldn't be visible, Scrat being at the bottom of the ravine he just fell in. (00:03:15)
Chosen answer: He failed at hunting and believes staying with the herd has damaged his predatory instincts.
BaconIsMyBFF