iPad and iPhone screens are made of toughened glass. While this makes them very resistant to damage, when they do break, they generally shatter into thousands of small pieces. One of the more peculiar features is that even after the glass has shattered, quite often the screen remains functional with a display still visible behind the shattered glass layer. It is possible to still operate the iPad with a pointing device, not a finger due to the risk of cuts from the glass shards.
Over a period of prolonged use, the battery packs in iPads and iPhones can deteriorate, just as in any other brand of Lithium ion battery powered product and the battery packs begin to swell from gassing from the materials inside starting to break down. A certain amount of gas is generated during normal charge and discharge cycles and the batteries are built to cope with that. As the battery ages, or is operated under stressful conditions, high altitude, high temperature, high discharge rate, more gas may be generated than the battery was designed to cope with and the battery pack begins to swell.
The first signs of battery pack swelling may be slightly discoloured patches on the screen of the display. The iPad or iPhone will feel hot to the touch as excess heat is being generated. The screen display may lift from the casing along one edge as the adhesive sealing the screen to the casing is forced apart. If the adhesive does not separate, stress will build until the glass of the screen suddenly ruptures. This is a very rapid event that may appear "explosive" in nature, but it is exactly the same process as occurs when a toughened glass car windscreen shatters. There is a bang and the glass fragments into innumerable shards.
When an iPad is operated with the GPS module activated, significant current is drawn from the battery pack. Most users find that an external supply is needed to keep the iPad operating for any reasonable time under these conditions. The iPad certainly gets warm to the touch.
We don't know how old the device being used was, or how long it had been used in GPS mode. It is reasonable to assume that this was a common practice given the circumstances described. High altitude, hot conditions, a battery being heavily discharged and kept going by an external power source. The rarified air at altitude also reduces the efficiency of thermal cooling. All of these things factor to make screen failure increasingly likely as the device ages.
This is not an unexpected failure mode and it is probably a fair assumption that the iPad was being operated outside of the manufacturers guidelines. iPads and iPhones are not hermetically sealed, so the effects of altitude alone are effectively harmless apart from reduced cooling efficiency.
If anyone wishes to demonstrate just how much power drain portable devices suffer when using GPS functions constantly, just use a constantly updating mapping program, or one of the popular GPS location based games such as Pokémon Go. A fully charged phone will completely discharge the battery in about two hours, an iPad may soldier in for an extra hour or so given the larger battery. Under more normal conditions, these devices will operate continuously for ten hours or more, hence the popularity of external battery packs.
Considering the number of devices in use, these incidents are comparitively rare, but they can and do happen. It is important to always consider such a failure as a possiblity when flight planning and have an alternative means of navigation planned for and to hand should such a failure occur.