Haven't experienced this (EX)... but wow, good thing the sunshade was closed to prevent the debris from falling onto you!
The breakage pattern looks somewhat unusual; I'd think a more circular radial pattern would show up if it was damaged from an impact -? But maybe this is how tempered glass shatters when not laminated in the way a windshield is. Do you have extra tint applied to the moonroof?
Out of curiosity, I googled "spontaneous moonroof glass implosion" and was surprised at the number of articles/news on it. "The most common causes are minor damage during installation (e.g. nicked or chipped edges later developing into larger breaks), binding of the glass in its frame (e.g. stress from expansion/contraction), poor manufacturing quality (e.g. uneven distribution of air during cooling, contaminants, lighter/thinner weights)."
-
This Consumer Reports article has two visuals showing incidents by manufacturer thru 2017. Hyundai has unusually high incidents; Honda is below-average in incidents. But the total number of incidents has been increasing annually (maybe because of broader use in cars?) and spiked in 2014.
-
This ABC/GMA video shows types of damage from overhead impact and mentions the start of federal investigation in 2014/2015.
-
And this video has brief tips at end for preventing heat-related implosion.