I looked for threads involving dead pixels caused by screen scratches, and I found one thread from someone with an iPhone 6 that dropped their phone and received a few scratches and a dead pixel.
Either a significant (and I mean very significant) amount of pressure was applied to the screen or the screen was gouged so thoroughly that a foreign object damaged the display. Those are the only two ways that the glass could cause a dead pixel to form. It's rare, but possible that a dead pixel happened to form where a scratch was located.