it was strange that the board was working when there was no power attached, but the driver was being powered - "where is this power coming from??"
If you do a Google search for "AVR back-powering" you'll see this is a common issue with the ATmega. Another case that gets people is when they try to use the Analog Inputs to measure a battery's voltage. If the battery is connected directly to the pin, it will "back-power" the chip.