Once upon a time, during a guided study(or WIN, as Clarke calls it) Aidan Duncan(azduncan) was in Mr. Frost's room, doing some corrections on some Mathcounts tests when Bronzefur(or was it ZhaoPow, I forget) noticed his great spelling. On one of his targets, he wrote “Dunan” instead of “Duncan” as his last name. Thus, everyone on the math team started calling him “Dunan”. Some random people noticed that people were calling Aidan “Dunan”, and then they started calling him “Dunan”, too. So now, about half of the kids in Clarke know him as "Dunan".
Aidan made “Dunan’s Theorem” after that. More about that could be read here: https://artofproblemsolving.com/wiki/index.php?title=Dunan%27s_Theorem. In fact, “Clarke Dunan’s Theorem” was a team at the Acton-Boxborough Math Competition (https://abmathcompetition.org).
Then one day Bronzefur put DUNAN LEAGUE as pretty much everything on his profile, so all of a sudden Dunan League was a thing. It was a profile pic, status, etc. Then azduncan, andyxpandy99 and kevinmathz joined. After that, bollywood added it on his profile, too. Then, jlordhe made it his profile pic. It became a trend on AoPS.