Ever since the money laundering regulations came in in 2007 each bank has developed their approach to risk, and often used this an an excuse to dump clients they don’t particularly want. Either because they do feel they are risky, or because they don’t earn them money, because they don’t like their name, or for no obvious reason at all! Consumer pages are full of stories of having accounts closed down with no notice. (https://www.theguardian.com/money/2018/feb/03/natwest-closed-my-account-with-no-explanation)
So I think this is probably another example of a bank cleansing its client database in a clumsy manner and using Brexit as an excuse.