Similar situation:
iPhone 6, never dropped, 6 weeks old. In fabric pouch of a beach chair on a 75 degree day (nothing pressing on screen, no impact, no flexing) - at 11:30, screen was fine; at 12:30 pulled it out and screen had substantial cracks emanating from the thinnest part of the glass right below the home button. Immediately went to Apple store, was told to come back two days later (earliest available Genius appt). By that time, cracks were substantially worse. Genius insisted only hairline cracks covered under warranty, only option was to pay $109 for screen replacement. Asked to speak to manager who reiterated same thing: "We have no way of knowing what happened, so it's considered physical damage and not covered under warranty". I reluctantly agreed to pay for replacement but insisted they include in service notes that "Customer reports cracks were spontaneous - no impact, no flex, no thermal stress" along with specific info on the location and spread of the cracks. A day or two later, Apple requested a survey about my experience and I made my displeasure very clear - not with the Geniuses, who were polite and professional throughout, but rather Apple failing to stand behind its product. A different store manager followed up on the survey response and after extended polite discussion eventually reversed the charge.
If you have spontaneous cracks appear, I suggest you document it immediately and insist that the techs record your assertion that there was no impact. That way if a pattern does emerge indicating a manufacturing defect affecting some phones, you will stand a better chance of recouping the screen replacement cost. With the accelerometers in the iPhone 6 it seems like they ought to be able to pull a shock history looking for impacts, just like the moisture tattle-tales that belie water damage.
If you actually drop the phone, though - it's on you and at least Apple reduced the replacement cost vs. 4/5. It did appear that they were replacing a LOT of iPhone 6 screens. Even the first Genius I spoke to said he had shattered his iPhone 6 screen dropping it onto the counter at Starbucks.