It's not a bug. Sometimes, when there is a lot happening on-screen, asking the game to process a collision is a little bit too much. The game recognises it, and prepares the car in front to ghost in the event of a collision in order to protect itself. It's by no means perfect, but the alternative would not be pretty. It happens a little in DIRT 3 when you're pushing your system too hard, so I imagine the same thing happens here.