Found a bit of explanation. None of this is mine.
I've read that because the Glock was originally designed for 9mm it doesn't do as well with the 40 S&W in regards to safety. The following is taken from Wikipedia, though I've sen the same basic information elsewhere on the internet.
"The .40 S&W has been noted in a number of cartridge case failures, particularly in older Glock pistols due to the relatively large area of unsupported case head in those barrels, given its high working pressure. The feed ramp on the Glock .40 S&W pistols are larger than normal, which leaves the rear bottom of the case unsupported, and it is in this unsupported area that the cases fail. Most, but not all, of the failures have occurred with reloaded or remanufactured ammunition. Cartridges loaded at or above the SAAMI pressure, or slightly oversized cases which fire slightly out of battery are often considered to be the cause of these failures."