IIRC, Mr. Zimmerman said that while he was on his back, Mr. Martin saw his gun (which was in his holster/waistband), and made some remark like "Now I'm going to get you", and grabbed for the gun. Zimmerman did too. Guess who probably got it by the barrel and pulled on it while it was aimed at his chest?
This would support the idea that Trayvon may have had a hold of the guns's slide when it was fired.
Of course, it was a Kel-Tec, so it may have just malfunctioned on its own.