http://en.wikipedia.org/wiki/Overdraft
So overdraft fees are not given for declined rebills. If they get charged an overdraft fee it just means the customers balance is in minus after you rebilled them (sucesfully). It probably happens with people that have a creditcard linked directly to their bank account. So they can have a 1000 limit in their card, but if they only have 800 in the bank account and spent the whole 1000, they are 200 in minus, and get charged the overdraft fee.