amazon associates suddenly asking for billing verification

Nothing in that banner on its own suggests the account is under review. Amazon asks for identity, payment and tax details to be checked before paying out, and sometimes asks again later. Do it from inside the Associates dashboard itself rather than from any email link, and make sure the name on the payment details matches the tax info exactly, mismatches are what usually hold payments up. Until it's done, earnings typically just sit there rather than being lost. If you're worried, check the account notices section, an actual review comes with a separate message.