I'm certainly no expert, but the unsupported case head is most often touted as the biggest factor, it seems. I think .40 factory ammo runs at higher pressures than the other cartridges, leaving less of a fudge factor.
Factory ammo seldom causes problems though, according to some. Many of the kabooms are occurring with reloads, and a good percentage of them are with lead, non-jacketed bullets. Evidently the polygonal Glock rifling is susceptible to heavy cumulative lead fouling, which raises pressures over the top. The general concensus is to stick with factory jacketed bullet loads for relative safety assurance.
Edited to note: Posted this before reading Doug's latest reply.
Edited again to correct Doug's name
Cheaptrick: Wasn't there an animation of "How A Glock Works" in that Arfcom gif. thread you posted the other day?
That explains it better than I can.
Edited by RONK - March/23/2008 at 18:02