Declines can happen for a variety of reasons. When a charge is submitted to your bank, they have automated systems that determine whether or not to accept the charge.
These systems take various signals into account, such as your spending habits, account balance, and card information like the expiration date and CVC.
Since these signals are constantly changing, a previously successful card might be declined in the future. Even if all of the card information is correct, and you previously had a successful payment, a future charge can still be declined by a bank’s overzealous fraud systems.