Continuity mistake: At the beginning and end of the film, there is a narration who the subtitles label as Sitka, but at the very beginning, we see that the narrator is actually Denahi.
Continuity mistake: After Koda frees Kenai from the rope trap, the rope disappears twice from around his (Kenai's) leg: first when he is walking towards his brother Denahi (then the rope reappears around his leg as he runs away), and disappears again when he meets Koda again inside the glacier (only to reappear again when he snags it around a rock/ice chunk and trips).
Continuity mistake: In the scene where the bears are riding the mammoth, Koda is touching Kenai, but when the camera angle changes, they are at least a foot away from each other.
Continuity mistake: During "On My Way," a shot in the song shows Kenai and Koda in the rain. Kenai's fur is soaking wet, but Koda's fur seems nice and dry, which is impossible if they're walking in rain.
Answer: The bear injures Sitka right before he sacrifices himself. We don't see the injury itself due to the film wanting to keep its G rating, but we do see the bear attack and most likely maul Sitka, heavily implying his injuries are fatal. This is why he's hunched over and breathing heavily when we next see him as the bear is moving towards Kenai and Denahi.