It isn’t any of their business unless you want it to be so just be vague as hell. “I got another offer that is a good opportunity.” “I’ve decided to pursue other opportunities.” They’ll obviously ask questions and you can just remain vague. “I would rather not discuss specific details of my new job.”
Or if you don’t mind them speculating behind your back you can say as something like “I’ve decided to move on for personal reasons.” Just leave it at that and let them sit there and wonder. Is a family member sick and needs care? Are you sick? Did you get a different job? Do you hate everyone? Are you going to prison? Who knows because you aren’t telling them.
That’s only from one breach. I’d wager that at least 75% of our SSNs are out there since this is constantly happening.