The best theory
The best theory I've heard regarding glocks and cast lead bullets is that the lead builds up in the chamber/throat area (or flakes off and lands there). This causes the gun to be slightly out of battery (Glocks can fire out of battery), and ....Kablooey!!! Hammered guns have less of a problem with firing out of battery because the hammer has the effect of driving them into battery where as a striker will actually push the gun further out of battery.
You can shoot lead bullets out of a Glock. Just not many, and use high quality hard cast ones. Oh and make sure you wear safety glasses.