You call it a “syndrome” as though it's a bad thing.
At Keith Lee's Associate's Mind he writes about impostor syndrome, which is
"a psychological phenomenon in which people are unable to internalize their accomplishments. Despite external evidence of their competence, those with the syndrome remain convinced that they are frauds and do not deserve the success they have achieved. Proof of success is dismissed as luck, timing, or as a result of deceiving others into thinking they are more intelligent and competent than they believe themselves to be."
I have done enough psychodrama training, and talked to enough lawyers in truth-telling mode that I believe that, at least among successful male criminal-defense lawyers, "impostor syndrome" is the rule rather than the exception. (I don't have an opinion on the impostor syndrome on the distaff side, nor in more boring fields of law.)
So much of what we criminal-defense lawyers achieve is objectively the result of luck-getting the right case with the right facts at the right time, drawing the right jury panel, finding the right piece of evidence-that much of what is described as "impostor syndrome" is accurate self-assessment. Even beyond that, though, successful men in criminal defense are, I believe, more likely to think of ourselves as lucky undiscovered frauds than as brilliant lawyers receiving our due.
This should be no great surprise; Dunning and Kruger would predict this result. And there are certainly exceptions, but the exceptions are those that Dunning and Kruger might predict: unskilled people who overestimate their own level of skill, rather than the more skilled who underestimate their own.
I see the tendency of skilled criminal-defense lawyers to privately consider themselves frauds a feature, rather than a bug. Because someone who secretly thinks he's a fraud doesn't want to be discovered, ((Oops.)) the lawyer who thinks he's an undiscovered fraud doesn't rest on his laurels, works hard, and actively hunts down and eliminates possible ways to fail-all behaviors that you want from your lawyer.
It's not that we have something to prove, but that we have something to conceal. And as long as we're concealing our lack of skill by acting as we would if we were highly skilled, we might as well be highly skilled.