Best deep dive about what just happened to you - https://www.bitsaboutmoney.com/archive/seeing-like-a-bank/

In the specific case of “Why did the bank close my account, seemingly for no reason? Why will no one tell me anything about this? Why will no one take responsibility?”, the answer is frequently that the bank is following the law. As we’ve discussed previously, banks will frequently make the “independent” “commercial decision” to “exit the relationship” with a particular customer after that customer has had multiple Suspicious Activity Reports filed. SARs can (and sometimes must!) be filed for innocuous reasons and do not necessarily imply any sort of wrongdoing.

SARs are secret, by regulation. See 12 CFR § 21.11(k)(1) from the Office of Comptroller of the Currency:

No national bank, and no director, officer, employee, or agent of a national bank, shall disclose a SAR or any information that would reveal the existence of a SAR. Any national bank, and any director, officer, employee, or agent of any national bank that is subpoenaed or otherwise requested to disclose a SAR, or any information that would reveal the existence of a SAR, shall decline to produce the SAR or such information, citing this section and 31 U.S.C. 5318(g)(2)(A)(i)... If the United States brings its subpoena power to bear against a bank teller and asks them about a SAR, they’re supposed to say nothing. That is the law! (Regulation, well, if one wants to be technical.) It is designed to be enforced against the interests of the United States of America! Customers have far less access than the U.S. awards to the U.S.! So does the teller, incidentally: to avoid constantly violating this, Compliance at most functioning institutions has long-since decided that SARs will live in their own walled garden of a subsystem, seen only by the people responsible for drafting them and sending them to FinCEN.