A failure of the Glock company more than anything else...they shoehorned .40 into a gun built for 9mm in an effort to beat S&W to the market of bringing a pistol out to go along with the .40S&W cartridge. MOST don't have issues, but there have been SOME issues with .40 cal Glocks (hence the change to a heavier, dual spring recoil spring in Gen 4 Glocks)
Glocks were designed for 9mm. As I understand it there was then a later design specifically for 10mm (the Glock 20). And then, if the history I researched is correct, the Glock 20 was adapted for .45ACP. Hence you don't get the issues of a 9mm design going up to .40 when your design, built for 10mm, is adapted to .45ACP.
It's a little of this. Although
the entire point of the .40 S&W (besides backing off of the post Miami shootout FBI 10mm project which some felt the stout recoil was a training/practice hinderance) was to "
get it into 9mm sized frames and designs". So arguably Glock just did what the .40 was designed for.
And as Giga said, it's a little of the small space of unsupported chamber at the 6 o'clock position where the feed ramp interfaces with the chamber. A compromise Glock made in the early runs in favor of feed reliability over case support. And the .40's much higher average pressure as divided by case volume/surface area. SAMMI is 35,000psi, the same as 9mm does, but in a smaller case, which will give 9mm a thicker web to case wall ratio. So that 35,000 psi has significantly more case internal surface area to act on in the .40. The .40 is a
much less forgiving cartridge, in any pistol. Even it's truncated conical shape in FMJ was due to the need to get 180gr into the bullet, while not exceeding the OAL of 9mm...
And it's a little of people ignoring the "NO LEAD BULLETS" rule with polygonal rifling, and over-pressuring their Glock, then lying about why it KB'ed.
And it's a little of Glocks being so popular. If one particular car was in the majority of crashes, people wouldn't automatically think it's the car, they might reasonably believe it's just the odds. Or conversely, people ignore simple statistics, and think "there's some kind of special problem"
And it's a little of gun culture/internet rumor. When a Glock KB's it's automatically "part of the pattern" in people's minds. Whereas every other KB in revolvers or other autos is dismissed as "some sort of random freak accident" etc.
So it's a little bit of everything, IMO. The whole "Glock KB problem in .40" is 20% the design/unsupported web/feedramp issue. 20% how unforgiving the .40 is in terms of pressure/surface area. 20% it's popularity and resulting statistics. 20% people lying, screwing up their ammo and cleaning practices. And 20% internet rumor mill/bashing.
In the breakdown, I say it's 20% Glock's design choices. 20% how finicky the .40 is, and 60% BS, stupidity/negligence, rumor mongering.
