Whether you give a reason or not if the lodgement is considered by the cashier to be out of character with your usual transactions there is a chance of it being flagged, that is what their money laundering training will have them looking for. Someone who regularly lodges larges amounts of cash may not be questioned as the bank will already have looked at the reasons and decided if it was within the norms of their account and business to have large cash amounts. Giving no good reason increases that chance. I would have thought 20k was well above the amount that would trigger interest in the transaction if no suitable reason was given.