Banks use various means to detect potential fraud and money laundering. Some of your past spending 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 laws. Banks are regularly inspected and audited to verify compliance with federal laws. Your account may have had an irregularity with your ID, Social Security Number, address etc. As noted above, banks typically 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 detail, finds and really anything about the restriction and verification.
https://en.wikipedia.org/wiki/Know_your_customer
https://en.wikipedia.org/wiki/Bank_Secrecy_Act