This sounds like your Apple Cash account.
Banks use various means to detect potential fraud and money laundering. Some of your past spending or transfer of fund patterns may have tripped their automated detection methods. Banks also are required by Federal law to Know Your Customers (KYC). These laws go hand in hand with fraud and Anti Money Laundering (AML) laws. Banks are regularly inspected and audited by the federal government to verify compliance with federal banking laws. Your account may have had an irregularities with your ID, Social Security Number, address etc. Banks may use an outside service to conduct the additional verification necessary to positively ID you and clear your account.
Banks are prevented from revealing the details and reasons for the restriction due to The Bank Secrecy Act. This federal law and supporting regulations prohibit bank employees from discussing the details, finds and really anything about the restriction and verification. The bank that issued the your Apple Cash account has determined your too risky and no longer want to issue and service your account and has terminated your relationship.
https://en.wikipedia.org/wiki/Know_your_customer
https://en.wikipedia.org/wiki/Bank_Secrecy_Act