Did this guy double-charge one or more shells in that batch?
Here's a recent article written on it.
Syria Booby-Traps Rebels' Ammo
New York Times reports on 'dirty trick' of war
(Newser) – Syria has resorted to what the New York Times calls "one of the dirty tricks of the modern battlefield"—booby-trapping ammunition used by rebels. As war correspondent CJ Chivers explains, the government puts the sabotaged ammo on the black markets where the rebels get their weaponry. It's not exactly a precision tactic, but it works. Chivers talks to one injured fighter who tried to fire his rifle at government forces, only to have it explode in his hands and seriously injure him.
Before piling on Bashar al-Assad, know that the US does the same thing against the Taliban in Afghanistan, and it also employed the tactic in Iraq, says the Times. In fact, it's possible that Syria got some of its booby-traps from the latter country. The practice even goes back to World War II, though it's rarely well-known. “The problem with them is the same as with land mines,” says one arms expert. “You can’t be sure who is going to pick up and try to use the spiked ammunition.”