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