The most likely candidates are satellites, airplanes, and meteors. In general, the streaks are caused by anything that appears to move relative to the stars. Stars, therefore, remain fixed points of light through the duration of the exposure. On the other hand, an object passing though the field of view against the starry background will expose the path which it took. They often leave two or more parallel streaks.
Satellites leave a faint, but very straight line.
The satellite's path line may vary in brightness due to it rotation and reflecting characteristics. Those steps can be used to approximate the satellite's velocity. Meteors are more unpredictable in their appearance. They often leave a very short path line with varying magnitude. If you see a streak that has a definite beginning and end within the field of view, it is likely to be a meteor, because satellites and airplanes rarely appear out of nowhere and disappear just as fast.