I don't know if the V20 camera glass is tempered glass, but tempered sometimes breaks without any obvious cause. It could be due to glass edge damage or surface damage from handling or from an inclusion inside the glass like a nickel sulfide stone, however this type of inclusion occurs very rarely. If you have a stone inclusion it can grow over time with temperature changes. The problem is that this rare and it is impossible to prove that it is the root cause of the breakage. If the glass is too tight inside the V20 frame it could also cause it to shatter.
More than likely most of the V20 and other phones that exhibit this problem are actually caused by the consumer without his knowledge. The giveaway in most cases is the pattern in which it breaks.
In the Jerry Rig Video he clearly caused it by stressing the glass, although it was a delayed reaction. In his particular case he took the protective plastic which made it easier to damage the glass.
I will be doing some test with a friend of mine who has access DOA phones that that have to back to the manufacturer in which we will deliberately check to see how easy they breaks by pressing a key up against it. I'm hoping to try a few phones.