On to your question: It's hard to pin down the exact root cause, but it seems (in my case at least) to stem from being an outsider -- a y in a sea of x's. When everyone else doing your job has a specific attribute that you don't, you can start to wonder whether you really belong in that role. The literal feeling is, "One of these days, I'll say the wrong acronym in stand-up, the rest of the team is going to figure out that I'm just winging this Agile Scrum thing, and that will be that."
That can mean being the only physicist in a company of CS grads, or the only woman on an all-male engineering team.
Being in a job where you're always learning, received no formal training, and where the expectations are fluid can amplify the feeling, since there's no yardstick against which to compare your performance. If you're a fast learner, it can be hard to believe you've gotten as good at something as people who had years of training/have been doing it for years.
Here's a group of people. Compared to each one of them, I know less about something. It's really easy to go from that to feeling like I know less about what I'm supposed to be doing than everyone else.
I don't really experience this, I suspect because of arrogance. I'm not sure that's really an improvement, though...