Being that the case, if it does it again, before pulling back the bolt handle to eject the round, look through the slot in the receiver at the back of the bolt, and see if the hammer followed the bolt. If it did not, then you'll see the end of the firing pin, but if it did, its hard to miss the hammer.
For the hammer to follow the bolt, the rear disconnector hook would have to not catch the hammer after firing. Check to make sure the hook spring is good, and that it works freely. Also, check the teeth on the hook, and the notch in the top of the hammer that the hook catches, for being rounded over, etc. It's job is to catch the hammer, with the trigger pulled, and not allow the hammer to follow the bolt. The front hook is the sear, which is used to fire the gun.
When the gun fires, and the bolt goes back, the hammer is mashed down, and the rear disconnector hook supposed to catch the hammer, at the rear. When you let off on the trigger, the sear hook at the front, catches the hammer, as the disconnector hook releases it. Then, it is cocked, ready to be fired again.