Let me share some interesting stuff.
I replace around 5-10 iPhone displays per day and I started to notice certain things about this.
How many they come in, depends on the weather (rainy day? "cracked screen with water damage" day), after weekends or long weekends (like Monday is crazy), after national holidays and season (winter is worst, probably because of people using gloves, have to take them off etc then there it goes).
Also there are types of people who can manage to be on the call while holding the phone up against their face with their shoulder while doing something else that requires two hands. That's a ticket for a screen replacement.
Many people (for some ridiculous reason) are still holding their iPhone with one hand while driving with the other. They usually end up dropping it between the seats near the handbreak (or faceplant the phone on it).
So it all comes down to handling the phone WHILE handling other things.
Don't do those I mentioned.
Funny story:
Last Saturday a customer came in with a 6 Plus that's screen was spiderweb-cracked and with the phone being bent.
He wanted a glass screen protector and a high-end Speck case for this phone.
We were explaining him the stuff about the screen protector on a cracked screen but he kept insisting. "So that it won't crack further!!44!"
You can guess the screen protector was all bubbly because of the cracks
He walked out spending around $70 for a case and screen protector for a bent, cracked iPhone 6 Plus.