I feel like this could happen easily if you force yourself to outline the whole thing from the get-go and stick to it relatively 'rigidly'; your brain will continue to chew on alternate ways to structure your story but you're not modifying your outline to match, so it comes out when you do the actual writing
Maybe that's not how it happened with you though, idk XD
Hm, now I'm feeling compelled to think about how this applies to my slice-of-life:
No more storylines worth exploring? Exhausted everything you can think of to write about those characters? Nah, I'm defs planning to write more stuff after bitwam, I've just decided to make them into sequels rather than part of the same 'story'
Main character overcomes a problem they've had since the beginning of the story? Entering a new life stage where they aren't going to be interacting with the other characters anymore? Well, there wasn't exactly a problem established at the start of the story ... I guess the first 'arc' marks the establishment and resolution of one problem? And my MC is still interacting with the other characters after the ending ...
I guess for me, it's a mixture of 'MC entering a new stage of life', and also structure; my comic is split into 4 'arcs' of similar length, all to do with a different character, but the stories I planned afterwards break that pattern. That's probably a big reason for why I'm designating the end of Arc 4 as the end of bitwam as a whole 
Does this generalize though? I also have other stories I plan to write that start off as a sort of 'episodic' thing but cumulates into longer arcs, which is sort of 'breaking the pattern'
But I still consider them part of the same story. Probably because the longer arcs aren't standalone and require the previous episodic stuff as context/buildup.
So in the end, I guess it's a mixture of factors, and also somewhat story-dependent XD