Why do we get Imposter Syndrome when we start a new role?

With the recent Covid restructuring at my workplace, I was moved under a new manager. She gave me my first big project, and I was so stressed that she was going to suddenly discover that I sucked at my job and was a total fraud! Classic Imposter Syndrome, doubting myself even though I'd been in the job for more than four years. So why do we get it and how can we stop?  See omnystudio.com/listener for privacy information.