Having read through the BBC report, it would appear HSBC became a victim of its own size. The report makes an odd statement though:

"But the reports failed to include key facts about customers, including the ultimate beneficial owners of accounts and where the money came from."

But in US beneficial owner's info was not required until recently ( https://www.fincen.gov/resources/statutes-and-regulations/cd... ; https://www.53.com/content/fifth-third/en/business-banking/m... ).

Granted, SARs get a little special since banks ( even internally ) are very tight-lipped about what SAR processing for legal reasons. So depending on the team, accounts may be investigated further as the situation warrants, but it was not a requirement of any kind as the article seems to suggest ( though it is phrased carefully enough to avoid saying that ).

Having said all that, I don't know if it is true at all banks, but you would not believe the wrangling needed to close an account. The times I saw it happened, it tended to be a clear case of fraud against the bank in individual cases, and some major issue with business accounts ( commonly now referred to as 'derisking' ). Normally, account officers, various VPs will fight tooth and nail to keep their favorite customer in ( and they should, but I want the reader to understand the issue is not as simple as BBC made it seem ).