Usually when there’s no reason provided it’s because an account tripped some anti money laundering rule(s). Someone in a compliance team then reviews the account and makes a determination.

By US law you can’t tell the customer that you’re closing their account because you thought it was “suspicious.” You just do it and then submit a Suspicious Activity Report (SAR) to FinCEN.

I can see how a high net worth individual could have account activity that looks like money laundering, but given his long history with the bank would be surprised if this decision was made automatically or by some junior analyst. It must have been escalated to someone senior, and they must have known who he was and still made the decision.