Obviously… The Aliens beamed down to take a leak behind the bushes. The driver of the car was startled by the sight of Aliens taking a leak behind a bush (not to mention the hovering saucer). This caused the car driver to drift into the oncoming lane, where the unfortunate motorbike rider met his abrupt demise. (Note, that if the arm in the grill of the car is supposed to be the one missing from the prone rider, then it is the wrong arm.).