Unless there's some bug in your transaction processing system causing it to leak excessive amounts of data on why the transaction was denied it makes no sense f

Unless there's some bug in your transaction processing system causing it to leak excessive amounts of data on why the transaction was denied it makes no sense for checkers to use it over their own merchant accounts. (And typical checker traffic is more like thousands of transactions rather than tens) It seems much much more likely that someone was just trying to card some candy.