Overwriting of unsigned driver in win2k3

Our QA are testing my unsigned driver on win2k3 SP2 x64 box. The driver is a function driver for an HBA that windows already has a built-in driver for. Mysteriously, after a power outage, the box booted and automagically loaded the built in driver for the devices my driver was handling. Checking the setupapi logs showed that windows was handling the devices as if they are new, and therefore didn’t remember that my driver drives the devices and decided to load the default “better” in-box signed driver.
Can anyone explain howcome windows did that after my driver was already driving the device.
Needless to say, attempts to recreate including reboots and power-outage didn’t reproduce the problem…