No, it's not the tracks. It has to do with how the video clip is created and played back. Movies shot on film and progressive video footage are the easiest to understand. Each frame of the movie is record as a whole and played back as a whole. However, interlaced video is different.
With interlaced video, each frame is divided into two half frames called fields. One field records the even numbered rows of pixels (called the bottom field because it contains the row at the bottom of the frame), the other records the odd numbered rows of pixels (called the upper field because it contains the row at the top of the frame). The two fields are not recorded simultaneously. They are recorded one after the other. When played back, the two fields are displayed one after the other and you see the complete frame.
The two fields need to be played back in the same order as they were recorded. If they are played back in the reverse order, the timing difference can show up as motion artifacts in the image. Where there is motion, the image can appear fuzzy or streaked and resemble the teeth on a comb (called appropriately enough "combing"). Interlaced standard definition video and high definition video are recorded using opposite field orders. If you mix the two together in the same timeline, you need to make sure you've addressed the field order of each clip to make sure all clips match the order used in the project setting.