Complex sentence: There have been some reports that the Disney film describes this as an actual suicidal action by the lemmings, but the narrator in the film states that the lemmings are likely not attempting suicide, but rather are migrating and upon encountering water, attempt to cross it.
Split into simpler sentences: There have been some reports that the Disney film describes this as actual suicidal action by the lemmings. However, the narrator of the film states that the lemmings are likely not committing suicide, but rather are in the course of migrating, and upon encountering a body of water are attempting to cross it.