People like to make jokes, and their jokes are usually pretty dumb. Sports fans really like to make jokes about the towns from which their opponents hail, even though every town is pretty much the same. Every town has rednecks, for instance.
This ramps up even further when we're talking about countries instead of towns. So when the United States lost to Japan in the Women's World Cup Final, so many sports fans reached for the most obvious USA-Japan joke that "Pearl Harbor" started trending on Twitter.
Twitter wisely adjusted its Trending Topics list to include "Congrats Japan" instead of any World War II-related topics -- including the equally charged "Japs," which also flared up at one point.
There was a weird, self-congratulate-y element to both terms staying so visible, though, as pretty much everybody decided to weigh in and express embarrassment at "Pearl Harbor" trending on Twitter. Which, of course, kept "Pearl Harbor" trending on Twitter.