Aside from a bad batch of parts or an odd bad part in a batch of good parts, my bet is that high volume dry fire practice is a significant contributor to a broken TRS. In contrast to live fire, with dry fire, the only limitation to trigger pulls over time is the available time and your finger getting tired.
I saw a guy build a contraption hooked up to an electric motor once to pull the trigger for him to smooth out the action. Shortly after hooking his new CZ up to it, the TRS broke. The machine was running no faster than you could pull the trigger manually, but it never stopped. I assume that in some cases the spring heats up to a point and then fails.
This seams to be in part why some folks have a practice pistol and another match pistol of the same type and configuration.
I have spent enough time with CZ 75 variants to find some of their weaker points. Ill bet that if i had spent an equal amount of time with another platform i would find some of their weaker points as well.