Answer:
Review your contract with the customer; i.e. the documents signed at account opening. It probably gives you the right to close an account unilaterally, with no explanation. There have been numerous threads and even webinars on how to terminate a customer relationship. The path common to every conversation is that the more you say, the more messy it will be...
Just cite the language in your contract and give them a date by which the relationship will be terminated. They know why you are closing their account. While they may be curious as to how you know their account should be closed, you are not obligated to share information with them.