I think it's pretty much near impossible that it is going to be the extractor, but one thing I didn't see (didn't read too close) is which generation Glock to which this problem refers. I have five Gen3 Glocks and I seriously doubt anything like this is going to occur with either the extractor of the ejector on the Gen3's. Not outside of the realm of remote possibility but extremely remote for certain. The design of the Glocks I own is such that the extractor would have to break and then get trapped between the striker and the primer to discharge a round. And the ejector does not come into contact with a fresh round either as it moves out of the magazine or when it enters the firing chamber.
Can't speak for Gen4's and I do understand they have had a few wrinkles.