First off a little background.
Perfect Cherry Blossom is this shooter game for PC made by Team Shanghai Alice as part of the
Touhou Project series.
The star of this fad is
Alice Margatroid, the stage 3 Boss.
Now along comes IOSYS OS, a music company that does remixes of video game, Doujin, and other songs. It typically promotes some of its songs by creating flash animations of it.
(Be warned, you may be driven insane after viewing the following videos, or at least have the song stuck in your head for a good while, I dare you to watch every single one of the videos below to completion though, lol)
IOSYS OS remixed the stage theme song of Alice Margatroid,
which starts 1:30 into this video playthrough of Perfect Cherry Blossom, and made a flash to promote it called:
Marisa Stole The Precious ThingSomehow this video became widely popular (or infamous depending on how you look at it), and was widely imitated, which began this IOSYS fad.
There are many others but mostly crap.
lolfad, that is all.