Part of this is that while under investigation they are not allowed to talk to you, and have no motivation to act quickly. I have read in a lot of cases, once they start the process it is very unlikely it will end well. You will, of course, get your money back (assuming you can prove everything they want).
They will close your account and the damage is done regardless if you are guilty of something or not. Quicker for them to close an account, then carrying on investigating a backlog of cases.
Of course, at some point, they will lose customers from a bad reputation but can take years before this becomes a mainstream problem.
I think the odds are small of this happening to you by accident, but it is happening.
This problem has not happened to me but did to a friend and I had to loan him money to pay his rent, bills etc. Took 3 months to resolve. Was with Halifax bank.
Personally, the main problem I have experienced is around credit cards and reducing limits when you pay off lumps sums. Worse still - I had a credit card closed and they took the money directly from my savings account as part of their T&C ability to garnish money from any other source linked to the bank. There was nothing I could do about it.
Lesson learned NEVER have a loan/credit card in the UK with the same bank as the one you have savings or your salary goes into Also, NEVER have your personal accounts and business accounts with the same bank!
The other comments on here about having multiple accounts for businesses can be a little tricky as some banks will not open a second business account if you already have done opened with someone else. e.g I had Lloyds business bank, tried Metro bank but they refused to open until I could prove Lloyds was closed.