So far as I can tell, no motive has been put forward for the attack.
The dead lived in Damascus suburbs. What was their politics? Were the merely chosen to die and are innocent of strong stated allegiance to either side? Who would benefit from the deaths?
This area of Damascus has been attack before.
From the article:
“For months, the regime has been shelling these neighborhoods northeast of Damascus, explains Tony Badran, research fellow at the Foundation for Defense of Democracies.
My response to txrangerette agrees with your assessment. I also added the point that the defecting General from Syria had been killed recently.