The most likely culprit is the AVS (Address Verification System) settings at your merchant account.
If you log in to your merchant account control panel, they often let you set the sensitivity of the AVS. This is what their system uses to attempt to verify a card is valid.
Set to it highest level, the AVS will not allow any cards which has a mismatch with the billing address or if their bank is unable to confirm the address is valid at the moment of the transaction. Some banks can't even confirm that instantly so those would be declined as well.
You can often set the AVS to be less sensitive. Some site owners turn off most AVS checks to allow as many transactions through as possible. If you are not sending out expensive physical items then it may be less of a risk for you. If you started seeing occurrences of fraudulent billing attempts then AVS could be increased.
You may need to contact your merchant account gateway support to help with this.