Remember when you go to Yellowstone National Park that though you might be observing the suggested distance between the wildlife and yourself, there's no guarantee that the wildlife themselves are doing the same. Just look at this video filmed recently in Yellowstone that shows a bear using the boardwalk as a handy shortcut. And, by extension, following a family!
As the folks that filmed the video say, "My wife, son, and I happened upon a big bear at the grand prismatic spring in Yellowstone that wanted to follow us on to the boardwalk for good ways before he leaped over the railing and sauntered off. No injuries but he was way closer than the recommended safe distance. About 15-20 feet from my wife at first."