We're not sure either! 

It could be because of your banks fraud protection, or possibly because you recently made changes to your billing information. Our payment system (Stripe) relays reasons for declined charges directly from your credit card provider. If all the information seems correct, your best bet would be to contact your bank to inquire on the decline and to ask for future charges to be accepted. We currently do not offer alternative payment methods. For more information from Stripe: https://support.stripe.com/questions/why-was-a-customers-charge-declined