Wikipedia page has some explanation.

There's also this gem:

In October 2016, McDonald's decided that Ronald McDonald would keep a lower profile as a result of the incidents.

That said, a 16 year old was killed in relation to these incidents, so not completely fun and games.

Wasn't it the "killer clown" prank timeline?

Not sure for other countries, but in mine it was just peoples disguising in clown for fun or doing prank on youtube, no real harmful acts.

It was what every cultural thing seems to be turning into: internet pranks/memes. Someone saw someone do it, did it themselves, when more people did it, more people did it. Simplest explanation is always the most likely.