I use an online bank which routinely asks about substantial withdrawals (check one online questionnaire pops up on my PC). I believe it is related to their marketing goals. One of the alternative responses: "I found another financial institution with a higher interest rate."
As I recall, one of the responses was "I needed to pay taxes", but I don't think there was an alternative for "romancing a Nigerian Prince".
If I so desire, I can choose not to respond.
I understand that banks are required to report withdrawals over a certain amount to federal regulators. There is no expectation of privacy due to the unblinking eyes of "the war on terror" and anti-money-laundering.