I've been trying to decipher a pattern to this and have come up with only one so far;
My partner drives the car, uses her key, and selects an album from the USB.
I jump in the car with my key, and it will be playing the album she selected during the previous drive.
I drive wherever, stop the car for a short time, then when I start it again, again with my key, it's playing the album I selected last time I was in the car.
The key used to start the car seems to have an effect in some completely random incoherent and useless way.
Other than that it's more or less random what you'll be listening to when you start the car except for the fact it's almost always something that's been selected recently.
It's not usually a big problem for us, I'm often undecided what to listen to anyway so it's almost a positive not knowing what'll be playing on any given startup. Most annoying scenario however is when you get in for a drive, select an album that's really hitting the spot, stop briefly for fuel/a drink/whatever, then upon re-start it's playing something completely different and you have to wait for the indexing to finish before re-selecting what you selected in the first place.
The fact that some people don't have this problem suggests there is a solution/workaround. Would love to know details of those not experiencing this - build date, usb stick brand/size, last update, anything really...
Gotta love MyLink.