It was likely from stresses from unequal force applied at different places on the glass.

It was experiencing a shearing force due to more weight on one side than the other, especially since he was holding it on the glass itself in the middle of the sheet.

The way he could've avoided it is by having two people lower it, holding on either side. Glass is inflexible, so it's especially fragile to shearing forces, so you need to be careful with it.