I've been going through some tracks and deleting some bits at the beginnings of clips, selecting the area in the time ruler. Sometimes when I let go of the mouse all the tracks become selected. It must have something to do with the movement of the mouse. I can't find anything in the (SONAR) manual about it.
 
Does anyone know what does this and how I can turn off this feature? 

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 find that I accidentally select all just clicking in the timeline to just pick a place to playback from while editing. Maybe the bakers can open that selection window a bit? I'm sure I maybe dragging just a small amount -  I'm probably being naive about how this would be calculated on a per track or project basis.  Or- maybe a switch to disable select all with in the timeline? I don't personally have a need to rapid select all from the timeline. If I did - I would be very purposeful to zoom out for a view of the whole project.

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.

See if this helps-
 
- First start by Shift+CNTRL+A to deselect all tracks
- Select the tracks you want to edit (delete, exc) using the track view or console view Track selection buttons.
- Drag in the TimeLine.
 
This will select only the tracks you have chosen.
 
You also may find using Right click+Hold to Lasso/Rubber band selections works as well.
For tiding up/house cleaning projects (seems to sound like what your trying to accomplish) Try using the Bounce to clips function. This will heal all clip fragments into one whole clip which also cuts down on space (from slip edits).
Then use the Save as/Clean audio folder function to purge the project of unused data.
 
 

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.

Good to hear.
Cheers