I'll repeat some advice I was just handing out elsewhere a few days ago: introduce your dog to new environments and situations on-leash, even when it is grown and a seemingly experienced hiker. New settings, with new smells and an unfamiliar 'feel' can stress a dog, and a stressed dog is not a happy, friendly, predictable dog. Leashing it tells it you will be responsible for keeping both of you safe, so it can relax more.