The manual safety primarily stops the weapon from firing if the trigger is inadvertently pressed. In these cases, the trigger was allegedly not being pressed when the pistol discharged.

Aside from the manual safety (which is optional on this specific model of pistol), there are a number of passive safeties that normally must be overcome for the weapon to fire. Something appears to be failing in such a manner that the pistols are firing without being commanded to -- the trigger is not being inadvertently pressed, so the manual safety does not seem to be involved.