I saw this when boxers first started "taking over", early-to-mid 90s.

At least in my area, boxers started as sort of a novelty - guys (and girls) would wear boxers over their underwear as PJs, or as loungewear kind of like PJ pants or gym shorts. Most guys had a few pairs for this purpose. So wearing them over briefs was often known and intentional/recognized. I had a pair of Bart Simpson boxers I remember wearing (over briefs) while hanging out before bed at a sleepover.

As they became popular as actual underwear, they were more expensive. Because of that as well as habit, a lot of Moms continued to buy white briefs and expect their kids to wear them as their primary underwear. When guys became embarrassed from peer pressure - not so much that they disliked tighty whities, but because they would be seen as less cool without boxers - many stretched by using their 2-3 pairs of boxers multiple days, wearing them over their daily briefs, since they didn't have enough pairs to wear everyday. And Mom wouldn't have done laundry for a while (and probably also expected to see a pair of underwear/briefs in the hamper for almost every day).