I think the effect is caused at least in part by the many small branches that you don't see in the image. As you're moving along in the car, the dark twigs and small branches moved across the frame at various rates depending on their distance from the camera. Some points on the sensor happened to image various branches and twigs throughout all or most of the exposure, while other points happened to image sky the entire time, and still others got half branches and half sky. You were traveling horizontally, so branches that are horizontal or nearly so are most likely to be cause dark pixels, but there will also be points where different vertical and diagonal branches all happened to land at the same point as the camera moved. That explains the random, snowy effect.

Here's an illustration: Spread the fingers of both hands and position them in front of your eyes so that one hand is at arm's length and the other is 3 to 6 inches closer to your face. Now move both hands back and forth horizontally, looking through your fingers at the sky or other bright background. As you move, you'll see some areas remain dark and others are mostly light. The faster you move your hands, the more obvious the effect is. Vary the orientation of each hand and watch the effect change:

• fingers all horizontal: you of course get horizontal lines of light and dark.
• fingers all vertical: you again get bands of light and dark, but each band is caused by different fingers blocking the same area at different times.
• fingers at different angles: you get a splotchy effect.