I've both seen it with my own two eyes, and heard about it in the comments section here at RideApart. There are a lot of off-roaders, whether they're 4x4 drivers or UTV drivers, ATV'ers, dirt bikers, or others, that just don't care about staying on the trail, leaving it better, or generally acting like full-grown adults.
Instead, they pretend they're on a closed course, in their own homes, leave their trash and broken parts along the trail, and generally cause a ruckus for everyone out enjoying these beautiful outdoors. Not only is that not being good stewards, but it's also being trash neighbors and it gives the politicians and corporate interests attempting to take away those lands all the ammunition they need to point the finger and nab themselves a scapegoat as to why they have the right to sell these lands off.
Most of all, however, it just is bad manners.