I'm ticked by the PP who said "and there's no evidence that the person was ever abusive" as if the bank is ever going to publicly disclose the content of calls or emails with a difficult customer. From what I know of banking, I'd nearly put money on it that he’s said something rude or unpleasant to the staff.
Banks are businesses. They provide a service but they're entitled to decide they no longer want a relationship with a particular customer and decline to continue to do business with that person.
I used to work in financial complaints and it was pretty common for a bank to issue the 60-day notice if, for example: the customer had complained too many times (like 20 in 2 years, with several payouts for goodwill); if the customer had used abusive language in their discourse with bank staff (people doing their jobs shouldn't have to listen to abuse or ranting); complained too many times about petty stuff while opening a new account (bank is entitled to decide this means you'll be a difficult long term customer); were receiving income from unexplained or potentially unlawful sources or were dealing in crypto as it's a hiding place for money laundering; or the algorithms have flagged unusual activities on your account that are typically hallmarks of fraud - even if you haven't yet been investigated or convicted.
There are a number of reasons. At the end of the day, the bank is exposed to a lot of risk through the behaviour of their customers and they are entitled to decide that a customer behaviour that poses a risk (even uncrystallised) is sufficient to refuse to do business with that person while other free-market options are available to them.