... either someone else has your steam details and is attempting to buy stuff (contact steam to go through these failed purchases)
Or your bank is flagging the transactions as fraudulent and stopping them (contacting your bank will tell you whether this is the case)
My money would be on the top one. The bank details he's using are correct as he bought Mass Effect, the block was put back on after that, which means another invalid transaction was attempted which caused Steam to flag it as suspicious.