There are two ways stuff can get into the wilds. People and nature. Nature is simpler so I'll get that out of the way first. Once heavy debris ends up in the woods it's likely to stay there if it doesn't degrade.
The only natural processes that can shift big things are wind and water. Your examples are too heavy to blow (though this explains things like fertiliser sacks). Flooding can carry things that float quite a long way from water sources if the land is flat. Empty barrels really fall into this category. Wheels might but probably not tyres on their own.
People do strange things. Some people's idea of enjoying the outdoors is noisy and petrol-powered, even if the trail isn't really wide enough and even if it's illegal. Sometimes they crash or get stuck, and if the car is stolen they'll flee. I've seen whole cars abandoned in or near footpaths. The spare wheel could also come off if the car hits the ground in the right place. Oil barrels are unlikely to be carried in by joyriders but forestry contractors refill their vehicles' tanks from them and I've never seen a major contractor be diligent about clearing up unless they were forced. After a few years of rust it would be hard to tell unless you could see an end cut off, but old metal oil drums are used as fire containers. If someone had lived rough in the woods they could have brought it in. Other signs of their presence could be more subtle - they'd degrade, get overgrown, or be more portable. I've certainly seen signs of people living rough on public land in the UK, US, France and Canada.