Answer:
It's either tiredness or distractions
Step-by-step explanation:
Most Pups don't have too much anxiety. But they are curious animals and if not on a leash will most likely stop to look at something. And tiredness, because pups can't walk as long as they will when they are fully grown and might affect their growth plates in a negative way. Though all these questions are possibilities the 2 mentioned here are most likely to make a puppy stray from Billy.
Hope that helps! :)