You guys really should take a moment to check out this report from TrendMicro (PDF). You'd be surprised to learn how far crooks are willing to go these days to steal your hard earned money.
It could be that these are regular criminal groups that already had access to the bank’s network and eventually realized that they could hop onto the ATM network. In Ripper’s case, though, it shows that some of these criminals are specifically looking for the ATM network as a target and not stumbling upon it by mere chance. These gangs have both the inclination and the technical knowledge to target these machines over any other resources of the targeted bank.