Feel free to lock/delete this is it has already been addressed, I can`t recall and am too lazy to do a thorough search.
I have watched The Lion King many times, and there always seem to be new things to discover about it. Details you didn`t really notice before, or you may realize something new you hadn`t truly thought about on previous viewings.
This is the case with me realizing that Simba must have thought he caused the wildebeest stampede. Sure, Scar is the one who brings him down there, but Simba still puts all the blame on himself for needing to be rescued. What struck me though, is that Simba must have thought it was his fault that the wildebeest started stampeding in the first place. After Scar walks off, Simba is left to practice "that little roar" of his. He does, and one of his roars echoes throughout the gorge. In the next second, the ground starts shaking (the pebbles start moving) and the massive herd of wildebeest come running down the side of the gorge. For the cub, I feel it`s a logical assumption that the stampede is a reaction to the sound of the roar. Simba must have thought his roar scared the animals into running...
I am kind of emberrassed to say I`ve never truly thought about it like that until the last time I saw the film, I`m sure you`ve all realized it ages go.