Although I don't know the true answers, here is my best guess on the reason why people do these two things:

Tire stretch/wheels outside of the fender - I think this is mostly due to people trying to get too much of a good thing. For example, most people agree big lips look good so how can you get a bigger lip? Well you could get a widebody but a much cheaper way is to have some rediculous camber and stretched tires. Eventually, people take these methods and take them to their extreme. Slowly it creeps into peoples psyche that more stretch = better, more camber = better, etc. I bet a vast majority of people who think that look is so hot now, probably would have been scratching their heads when they first saw it. As with all fads, eventually people sober up to the reality that it really makes little sense and that functionality and beauty are highly integrated.

JDM - I think there are three main reasons for this fad. First, its simple backlash from the days of huge wings and mammoth body kits. Second, people think of Japan as an exotic authority in cars and therefore whatever they do is cool and respectable. Finally, people like belonging to groups. Following the trend allows you to be a member and is like a quick path to more friends. It's interesting that people are willing to label themselves as "new drivers" (Sashinoya badges) in order to seem more JDM which is so contrary to most car enthusiasts.