So something was pushing the trigger to the rear from the sides? I'd have to check all my passive retention holsters, but I don't think any have material that comes close to any part of the trigger. Even pins or other hardware that would be near the trigger. I'm gonna tighten down the hardware on them while I'm at it. . . 
If you check the left side of an OEM Glock trigger, you see a pin that acts as the pivot for the safety release. If that pin loosens enough, the safety toggle can fall out. What was catching on the holster was that safety pin. It was still in the trigger, but less than 1/2 way.
You can't see the problem in the video, because the hole for the pin only goes through the left side of the trigger. But, you can see the safety toggle is missing from the trigger.
I wish the person posting the video had taken a follow-up video, or taken time to record the exact cause. As it is, you have to read all the comments to figure out what happened.
He says this can happen in any Glock trigger, but I'm not so sure. The Zed trigger is aluminum. That gives a metal on metal contact surface with less friction.
The OEM trigger is a metal pin in a polymer tigger. Unless the pin hole is enlarged, it can maintain pressure and friction around the pin. Looking at my G17 Gen 4, that pin is seated pretty snuggly in the trigger.
If you have any holsters with retention caused by pressure on the trigger guard, like almost all thin IWB Kydex holsters, I guarantee the holster brushes up closely to the trigger area.
