When you deactivate your account, you can reactivate it again by logging in with your password. We'll then send you a confirmation email to make sure you wanted to reactivate.
If you got a "Pinterest Account Reactivation" email and you didn't try to reactivate your account, then someone else knows your Pinterest password.
You should reset your password to protect your account and stop these emails. Don't worry, resetting your password won't reactivate your account.