There have always been people scared of clowns, right? And if you saw a clown just walking around, especially at night, it might freak you out.
So, in 2016, there were some pictures going around of a clown wandering like a parking lot at night somewhere in Wisconsin or Minnesota, I forget which. This ended up being promo for some movie.
"Clown sightings" started popping up all over, though, with the clowns being places they don't belong and often acting suspiciously (trying to lure kids away, for example). I believe at one point someone got stabbed.
Anyway, there were nationwide and even international consequences - clown costumes got bought up, retailers stopped offering them, everyone with anything to do with clowns made a statement (including Stephen King), and Ronald McDonald was mostly pulled out of McDonald's advertising. People were genuinely scared for their children's lives that Halloween. Additionally, many schools took precautions - clown sightings nearby would result in lockdowns, and (in my area) students were forbidden from using outdoor walkways for the duration.
Some people thought it was a conspiracy, others thought it was promo for the 2017 IT remake, but it ended up just being random disconnected people trying to freak out the public.