I remember this happening to me when I was a broke college student over a decade ago. I had to pay some bills and buy some things, and I expected a single overdraft fee, as I knew my exact account balance and the order of the purchases. I remember being hit with 3 (maybe 4) overdraft fees and immediately called my bank for an explanation. They gave me some bs like, "We reorder transactions to make sure your large ones go through first since they're probably more important." It was obvious at the time what they were really doing and why, which led me to eventually close my account with them and go with a local credit union.