It’s often quite trivial for an experienced GM to safely assume that a specific event or outcome is going to happen. For example, if a typical group of heroic PCs are riding along a road and they see a young boy being chased by goblins it’s probably a pretty safe bet that they’ll take action to rescue the boy. The more likely a particular outcome is, the more secure you are in simply assuming that it will happen. That doesn’t mean your scenario is railroaded, it just means you’re engaging in smart prep.