Declines happen for a variety of reasons. When a charge is submitted to the bank, the automated systems decide whether or not to accept the charge. The systems take your spending habits, account balance, card information (expiry date and CVC) into account.
Sometimes even if all the card information is correct, a card maybe declined in future because these signals are constantly changing.
If you want more information about a specific decline or how to decrease the likelihood of a decline, you can read it here. - https://support.stripe.com/questions/why-was-a-customers-charge-declined