My husband and I just travelled from LA to NYC for a week. While in NY, without any drops, impact or other crack-causing situations, both of our phone screens cracked, two days apart; my phone from the right-bottom corner by the microphone and my husband's phone from the top-left corner by the speaker.
These phones are new; bought on May 31st. No drops. Both phones have had screen protectors and sturdy Speck protective cases since they came out of the box (installed from the Verizon retailer).
We can only assume it was the heat or humidity shift that caused both screens to crack.