Early to speculate, but could it be that one pilot was left in the cockpit alone and something happened to him (heart attack for instance) and due to the rules implemented after 9/11, the doors are kept locked? If that's the case, I'd hope there were rules about not being alone in the cockpit.

anyone in the industry know how that works?

..is the plane not on autopilot anyway? also there is a code known to both pilots in such a situation to gain entry, all I can imagine is the one flying the plane locked himself in overriding the other ones code and flew deliberately into the mountain.
In order to prevent the other pilot gaining entry he had to actually do something, which removes him from bashing his head or having a heart attack, very strange if all this is true.

.......has our coach driver done a Poo'yet, without thinking about Gus!

..read that in Germany one pilot is allowed to leave but only for certain time, that is obviously going to change......today!

.......has our coach driver done a Poo'yet, without thinking about Gus!

..exactly I was thinking the very same, but these days very little can go wrong with the actual plane, it's us that ferk it up.

.......has our coach driver done a Poo'yet, without thinking about Gus!

Looking generally, often there are only 2 pilots on the "Flight Deck" but one can leave to use the toilets leaving only one in Control. Surely pilots are allowed to have a "Comfort break" on say a 5 hour flight to Sharm-El-Sheikh.

On longer flights some airlines would in that situation require a member of the cabin crew be present in the cockpit.

I have prepared one of my own time capsules. I have placed some rather large samples of dynamite, gunpowder and nitroglycerin. My time capsule is set to go off in the year 3000. It will show them what we are really like.