From personal experience, I know one type of crowd that leaves trash behind are those that like to booze it up big time at night and then leave that night or early the next day. I've picked up dozens of bottles and cans left behind as these boozers down on then casually throw it over the shoulder. These are younger kids doing this from what I can tell. They are also the ones that will play their music loud at night while they "party". Rangers should crack down on this behavior with heavy fines. The forest isn't for partying it up. The second type are just the lazy and thoughtless people. Annecdotal evidence suggests there is some cultural element to it as well but not exclusively.
What surprises me is that we as a society have adopted the use of plastic for everything and yet only just recently are we coming up with ways to handle the waste stream. It is with much irony that our "recycling" efforts for the last 30 years have involved collecting what we can then putting it on a ship and dumping it somewhere else. Did we actually recycle anything or are we just making some people feel good about it while ignoring the problem further? Might we have done better just to bury or incinerate it ourselves where we can control the method it is done in? Today there are modern recycling plants that can effectively sort single stream waste plastic efficiently and effectively to make the end product valuable to manufacturers but that is a relatively new development.
In my own perfect world, plastic wouldn't be used for single use applications. IE, anything at a fast food resteraunt should be compositible. Any single use consumer plastic product should be designed to break down within a few weeks of exposure to UV rays or salt water. We can put a man on the moon, (or could at least at one point...) how come we can't design plastic for our potato chips that disintegrates when exposed to sun or salt water?