My bank paid my largest check first and then the smaller ones. Doing so created more overdraft fees on my account. Why did the bank pay in this order?

You may write your checks in numerical order, but that doesn't mean the bank will post them that way. The same is true with point-of-sale or other electronic transactions: They don't necessarily post in the order in which you made the purchases.

When several items come to the bank for clearing, it can choose to debit them from your account in several ways. Many national banks are opting to post the largest dollar items first instead of posting the checks in numerical order. Often the largest check represents payment for rent, mortgage, car payments, or insurance premiums.

If your bank adopts this policy throughout its territory, it normally will notify you via your statement.