When a video from the original incident have surfaced someone here suggested that it might had been a hand-grenade. The hole seems to be on a right height. It wouldn't surprise me if this particular incident was accidentally caused by someone carelessly handling explosives.
Btw, I also saw a vid from a direct aftermath. It seems that two young guys were ripped apart. Now I'm even more inclined to believe that in this particular incident they accidentally killed themselves. Not that it matters for the general picture of the Syrian tragedy.