i think it's a variety of factors and while i can't say for sure if i've experienced the same (mainly because i gave up on trying to keep up with numbers) i'm sure it happens to a lot of folks on social media in general
in some cases yes there are people who follow that expect daily posting. this especially since instagram's algorithm forces that kind of activity just for a page to stay relevant. so when activity drops sometimes they'll unfollow.
other cases it can be because it's an account looking to get a follow back and when they don't get it they leave. it seems there's a good chunk of intagram users who expect a follow for follow but the response when they don't get a follow back is usually just an unfollow.
there's also situations where people just don't see posts (which isn't uncommon when people follow hundreds or more people) so when they come back they might not remember having followed or maybe lost interest.
i don't think it's worth blaming oneself since social media kind of forces you to grind and not everyone can keep up with it. posting everyday for some folks isn't really plausible or possible but i think a level of consistency usually is fine like having finished peices some days and sketchy stuff others maybe. Another thing I've seen some do is sharing work in progress stuff since there's an audience for that as well.
I think it also helps to have a backlog of art so you're not getting burnt out producing extra content just for social media. If you've got a habit of doing a lot of side/personal art that's something to consider holding onto to post. Or if you have a comic or even a novel little snippets or previews are a good way to generate content and also advertise what you're working on
personally i'm not as active on insta as i used to be since i'm a bit more cozy on twitter but these are things i feel tend to work or have seen in action. that and it's a bigger load off of my mind when i'm not stressing about numbers. I'm aware that some people prefer to track and see how their doing based on them which is fine it's just not for me