I understand what it looks like, but we have the guns from those incidents, and no one has looked at them an pointed to a defect, or reproduced them firing without the trigger being actuated.
A gun in a holster can fire when it is moved and the holster is poorly fit, incorrectly configured, or there anything caught in it like tail of a shirt, drawstring. Also, many police have a flashlight on their pistol, which opens up space quite a bit making it easier for things to get caught inside.