A couple of things:
1. The phone has Gorilla Glass 2, which is lighter and thinner than Gorilla Glass 1, which means it is more fragile.
2. The design of the phone results in the edge of the glass being very close to the edge of the frame. This leaves the glass closer to the point of impact.
Also, since the phone is made mostly of plastic, which does give and allows a certain amount of flex, this may be an issue when it is attached to glass, which is fairly rigid and allows a much smaller amount of flex. When he was putting the battery in and then snapping the battery cover on, if he was applying a larger than necessary amount of pressure, it could very well have bent the phone enough where the plastic gave and didn't break, but the glass did. Luckily the damage was when the phone was in their hands, not yours.
I have seen phones drop from waist high land flat on the glass and did not even scuff and other situations a phone drop from maybe knee level and shatters the glass. With any thing it is a matter of angles, how it lands, and where the weak spot is in the glass. Personally it is why I will always keep a least a tpu case and a screen protector on my phone. Because it is for that one moment when you least expect it and having a case helps add protection to my device. I had a rep drop my galaxy nexus when installing a battery and all it did was scratch the casing.