I think the one of the more common reason for imposter syndrome is that a lot of smart people constantly get told as children the "you're so smart/capable, you can do everything!" too much, and when you hear it enough times, it gets to you, so you think everything is just easy. And then when they start hitting roadblocks, instead of helping or explaining that it's normal for things to be hard and it's normal to fail, usually parents and teachers and whatnot tell them "Oh it's okay, don't worry about it, you're smart, you'll get it" and so they at first it works, maybe it just takes more time but they manage, but as things get harder and they still put little effort because "don't worry, you're so smart, you learn so fast/easy" and as they find out more and more things they don't umderstand or don't know they start to feel a dissonance, which builds anxiety.
And this is where I thinks it actually starts: at some points there comes a situation where they either share this anxiety with someone or someone notices their worry, and(at least from what I've seen from others) usually the response they get is something along the lines of: "Nah, you're just worrying too much, you're smarter than you think, don't be so down on yourself, you need to worry less", which, maybe I'm wrong, but I'm not sure telling someone that thinks he has a problem that he doesn't have a problem, helps their worrying.
And on one hand the amount if things they don't get/know/understand or fail at grows(cuz you can't just be good at EVERYTHING, so the more things you know about, the more things you don't understand) while mentally still being in that "Wait a minute, you're smarter than this, you should be getting this!" mindset that's been drilled into them, and so at some point the illusion shatters, and they start to think "Maybe I'm not so smart after all", and because they think they were wrong about their level, they feel like they have "oversold" themselves in the past and that makes any past accomplishments feel like lucky accidents instead: "If I'm not actually smart, the things I did manage to achieve must've been just accidental", which makes them feel like they've lied to themselves and everyone else when they "took credit for an accidents" and that their life is just a snowball of pretending.

Now, is that actually a cause or is it another one of my crazy 1AM ramblings? I don't know xD
I'm not an expert in any of this and I don't really know any psychology so hell if I know if that's how any of this works but that's just my theory of one of the reasons why. *shrug*. I've had this theory for years, but I don't know.
It at least makes sense to me, but not everything that makes sense is true soooo.

Anyways, wall of text is over.

Oh, and for anyone struggling with imposter syndrome: I just want you to know, it's okay to fail, and it's okay to not know shit, especially in the dev industry where every "insignificant" detail can have an entire rabbit hole of expertise behind it, nobody can expect to know every part of it. And it doesn't make you any less smart no matter how much you fail. Tnis shit is hard, so I hope you stay strong and I hope you succeed in whatever it is you're struggling with.
*Massive virtual hug* <3

Combatting imposter syndrome is all about being more realistic with yourself imo.

Not in the way you might think. By realistic, I mean you NEED to regularly tell yourself that you are doing your best - especially in the work or areas that can promote insecurities of “not being good enough”. Acknowledge that you are only human, that all of us are different, that all of us make mistakes, and all of us have different interests in life. That, and practice gratitude for your situation. Your interests and decisions lead your different paths, so might as well embrace, enjoy, and love your uniqueness.

That being said, I also think it’s important to do difficult things. I think @wisecrack said it the best in that “real learning feels like falling”. Like the uncertainty of the abyss causes the most anxiety. Next time you feel like you don’t belong, recognize and separate that feeling and reframe it as a symptom to your own self improvement process. Take that risk and do things that are uncomfortable in the pursuit of personal success.

There is like a dozen shows streaming on how actual impostors imposted the shit out of people.

I never claimed to be the heir of a machine that can make art out of deposits of a single drop of blood to dozens of people on dating apps.
I never said that I can double their bitcoins by harvesting carbon monoxide and recycling plastic into endangered fauna, expanding the mind of humanity.
I never got private-jet levels of money to do something I knew to be completely impossible from the beginning.

If I ever feel like an impostor, I could honestly say that make for a very lousy one.
I guess I've got impostor syndrome squared - I feel like I'm really bad at impostor-ing, but people could be thinking that I am better at pretending to pretend to be something I actually am.

Hits hard when you learn fast, and within 9 months jump from not knowing how to code to developing a whole integrated management systems webapp.

I second guess myself all the time.

Everyone has gone through it, at one point or another, for smaller or longer times, just once or maybe recurringly.

Just because it's happening to you doesn't mean that it's ONLY happening to you.

Chill out. No one "knows" what they are doing.