MOs have no parentage. What I mean is that there is no way to trace the source of funds used to purchase the MO so the bank has no idea where the funding source arose.
This means that unless they know the depositor very well, there is no assurance that the depositor is not participating in ML activity. No bank is going to tolerate the liability that will accrue to them if they are found to be laundering money.
So, any behavior that even hints at ML is going to be evaluated closely and tossed if judged to be risky.