Unintentionally selecting all tracks (Solved)

It could be your mouse.
But clicking or drag selecting in the timeline will select all tracks.
your better off dragging on the clip itself.

I guess the safest way is to take chuckebaby's advice and drag on the clip itself. 
 
However, I've almost always made selections from the timeline. The manual, in one of the tutorials, suggests dragging in the timeline as one of the methods for selecting an area in a clip to delete. There's no mention of the possibility of selecting all the tracks. In fact, I can't find it mentioned anywhere, either the manual or the Garrigus book.
 
Also, it's odd that it doesn't happen every time. I would like to know what I'm doing differently the times it does so I can avoid it. There are times when making the selection from the timeline is more practical for me than making it from the clip.

Thanks for your advice, chuckebaby. It inspired me to experiment.
 
Apparently, if you drag in the timeline with no tracks selected, it automatically selects them all. If you've already selected a track, it doesn't do that. When it happened to me, I must have selected the area of the clip I wanted to delete but hadn't yet selected the track itself. I guess that's what you were trying to tell me.
 
The cleanup I was doing was removing the little noises before the playing starts (putting the bow on the string, etc.) and the clicks at the end of the clips when I stopped the recording.
 
Thanks again for your help.