It's not really supposed to make sense, it that case it was I think one or a few pretty good artists who started drawing it a lot and it went viral. Powerpuff Girls and Samurai Jack had a lot of audience crossover and a meme was born. Who can say why the artists started drawing it, could be a lark or an in-joke, some what if idea, or just autism.
Usually these ships are either that (fan artists draw something a lot until it becomes a meme), a joke (Reptar and Heath Ledger Joker), or just really obvious like Jack Frost and Elsa.
A weird example outside of those was the fan game Mugen leading to ships like Rorschach and Ryouko from Haruhi Suzimiya and other crossover ships based on tag team combos that worked, so strange outliers exist, too.>>100799162
Gravity Falls first came out when Paranorman had active fans (can't even remember which came out first but close enough together). Both were about kids a spooky small town shit. There was a ton of crossover fan art and shit, Parapines was either what all of it or just some crossover ships were called, I don't remember.
That crossover stuff faded when Paranorman fandom became mostly inactive. It was a single movie and not a monstrous hit or anything so that's not surprising. But it the early days of GF fandom it was everywhere. It probably helped that people hated all the early season 1 ships on offer that weren't incest and people who couldn't stomach incest but just needed to "ship" things were looking for anything to fill the void. (Actually for that matter, the "only popular ship is incest" applied to the Paranorman fandom, too.)