EVO syndrome.
Back when Mario won EVO a few months ago, everybody was saying Mario was Top 5/3 for sure, that he was so amazing, and thus a lot of people starting using him that weren't before - especially on FG. Then people saw Mario is more complicated than he looked and dropped him.
Recently, MKLeo won Zero Saga mostly using Marth, and it was Marth's first major tournament he won, and thus now you have people saying he loses no MUs and people are coming out of nowhere trying to main him - again, especially on FG.
This sudden influx of Marth players is gonna drop as soon as people start noticing Marth is not easy at all. A lot of characters went through this. Off the top of my head, I remember Sheik, Diddy, ZSS, Mario, and now Marth.