They have specific laws that BAN them from giving detailed reasons for why they froze your accounts. Giving you a reason, or sometimes even hinting that there might be a problem, would be classified as "tipping off", which is a federal crime and can land even the lowliest finance worker in jail.

People need to read up on AML (anti-money laundering) laws. They are a bit insane.

Know them because I've worked in regulated financial companies for years and have had to do the mandatory training about a dozen times.