The period circa 1989-1993 in the
United States (perhaps elsewhere, but I didn't grow up elsewhere, I grew up outside
Chicago) when the youth fashion trend migrated from
tighty whities to
boxers. Originally, it struck me as an element of the college sports clothing craze at that age, manifesting as an inch or so of
college mascot-printed cotton sticking out past guys' shorts in warm weather. (Note that here, boxers were displayed
below the shorts, as opposed to the later trend of
sagging* one's pants to display the waistband underneath.) Over time, the mascots diminished in favor of the traditional solids, plaids, and yellow-smiley-faces-with-suggestively-placed-tongue.
I first noted them my sophomore year of high school, and by casual observation and anecdote they seemed to have become the de facto standard during my freshman year of college. Like any fashion trend, I have no clue what sparked it. All I know is that boxers weren't at all effective at concealing the signs of adolescent male hormones, so they weren't going to work for me... ;-)
* This looks really silly with white briefs underneath. *shakes head* Suburban ghetto-punk-wannabes...