To this day I don't know the reason and I asked why. The answer I got was that sometimes new accounts get FR'd. I personally believe is that it had to do with the types of accounts reported on my CR. I had Credit One, Orchard as open TLs with relatively low limits and on my app I reported an income of +100K. The numbers didn't jive I guess. That's my best educated guess.
AMEX is not the only card issuer that wil request financial documentation. They do it more frequently then others.
One of the triggers I know of or at least for me was requesting too many CLIs for 3 of my credit cards over a short amount of time. I had $75K in unsecured credit not incluing my charge cards. I can TOTALLY understand the FR.. However, I did not need to submit bank information. I simply allowed them to see my tax information.