I quite don't believe in his claims.
A reputable bank will not close accounts of it's clients unless violating some legal aspects of their financial terms also closing account means only freeze until further investigation. If does happen, then a client can go directly to the higher state of the law to sued a case. But prior to that, there will be exchange arguments from both parties since reputable bank didn't want to give dirt to their names just because of one person even it has a possession of millions amount of money. It''s against the law that a bank will close account "with no other reason".
If OP is really mad then he can give a shameful comments to that bank throughout social medias. Named that bank and spread the word of yours.
I had the same disbelief as you did because the bank was reputable, in fact, one of the biggest in the world. However things turned out far beyond my estimation. If you google "bank closed my account without explanation" you will find how many people were in the same boat. A bank doesn't need to wait until you commit a crime to enforce account closure. They can let you go anytime for any reason, even refuse to give your money back. That's why I was so upset. Also this does not occur only in United States.