Screens have been known to crack due to temperature changes, increases, decreases. What usually causes the crack is a microscopic hairline fracture along an edge, too small to see with the naked eye, but it essentially creates a point of "stress relief". This then coupled with the temperature changes causes the glass to expand every so slightly in an uneven fashion and that stress reliever decides to take off across the screen.
To help explain, when glass is cut, it is "scored" or essentially scratched along its surface, which creates a point of stress (a fault-line or path of least resistance). Then the glass is bent along that stress line and the hopes are that the glass will break cleanly along that line. Now, I don't think you created a score line on the screen, but you could have dropped it, even a short distance or bumped it along an edge, and though there was no visible damage, the stress fracture could have been created unbeknownst to you.
Also this can happen during manufacturing, so I am not saying it was you that caused it. If the glass is bumped on an edge in the assembly line before being inserted into the phone, that stress fracture could have been there all along. It doesn't need to be long, even a millimeter long is enough to set the stage for a catastrophic failure like you experienced.
Finally, batteries which can swell during charging or discharging can also create added stress and cause the same results.