There are two answers as far as I know. 1. The sending bank can reverse an ACH (electronic transfer) within a certain period of time due to fraud, death of the

There are two answers as far as I know. 1. The sending bank can reverse an ACH (electronic transfer) within a certain period of time due to fraud, death of the recipient, etc. (Up to 90 days in some cases.) By holding up the money, a bank protects itself from losing money on a reversal. If anything seems fishy, the receiving bank might hold the money for a few days. 2. The bank can get a little more float. Back when WAMU was going down, they started taking a week or more to clear large ACH's and deposits. Most banks don't hold innocent-seeming transfers for more than a day. The larger than usual transfers are the ones that often are delayed.