That's a bit confusing for me, too. It seems that some tracks anchor to one another automatically, though I haven't quite figured out what the pattern is. Here's what I do:
I have a video clip on video track one (the lowest track on the stack). I have a piece of music on audio track 1 (the highest one on the stack). I have a still .png with transparency on video track 2 and a title on video track 3. I want to move video track 1 later in the timeline, but to keep the music track to start at 0:00:00. So, I tap and hold the clip on video track 1. For some reason, video track 1, video track 3, and audio track 1 are all anchored together, so when I drag video clip 1 to the right, it brings video track 3 and audio track 1 with it.
To unlink or un-anchor the clips, I have to select the audio track and then tap the little "link" button (looks like a three-link chain) to the left of the timeline. Then I have to tap the clip on Video Track 3 and tap the "link" button. (When the tracks are linked, the link button is blue, when they're not linked, the button is white.) You can't just tap the clip on video track 1 to unlink the clips; you have to tap the clips that are attached to the clip that's on video track 1.
I hope that makes some sense. It's still kind of early here in San Francisco....