In my experience yesterday, it also did it going up, but in no way is it a desired effect. FREAKY is more like it.
 
Today I am unable to reproduce it in any project so I'm calling it a fluke that may or may not ever happen again.
 
I'm changing the title to more accurately fit the scenario.

It's back and reproduceable in several projects on both MIDI and AUDIO tracks.
 
What: Fader "slipping" on it's own to maximum or minimum level
 
Where: Inspector Console strip
 
When: Moving the fader to a different level while holding the mouse button down *only when song is in PLAY mode*, not when song is stopped
 
Why: You tell me

Not happening here with a wireless Logitech M510 ...

It happens here with 2 different Logitech wireless mouses (M187 & M325) and the built in touch pad.
I'm using an Alienware laptop 17 R3 running Windows 10 Home so it could be something in my particular system.

Did you try disabling the touch pad while you have a mouse connected?  I have seen some spurious cursor movements occasionally when both have been enabled on my laptop.  Mostly web browser stuff though, never Sonar.

Touchpad auto-disables when a mouse is detected.

In 2017.05 it also happens in Console View Bus & Master Strips so I've edited the OP.