I am speculating.
Here from CNN:
Hundreds of French firefighters and police officers are involved in the recovery effort in the Alps. Searchers have so far retrieved the cockpit voice recorder, one of the plane’s two “black boxes,” said Cazeneuve, the French interior minister.
The debris at the crash site is not characteristic of a plane that exploded in flight; it suggests the plane hit the ground and broke apart, said Remi Jouty, the head of BEA, the French aviation investigative arm that is leading the crash probe.
The cockpit voice recorder, which is designed to capture all sounds on a plane’s flight deck, is damaged but not beyond use, he said.
Investigators have managed to hear some audio from it, Jouty said.
Hollande said the outside frame of the second “black box” — the flight data recorder — has been found, though not the recorder itself. The flight data recorder stores a vast array of parameters about the aircraft.
The DFDR (black box) is located in the tail of the aircraft... can’t be disabled as far as I know (for exactly this reason). ADS-B data was received all the way down... that is broadcast position/speed/altitude. Easy to just not respond to ATC... no need to turn the radio off, just don’t reply. I’d not be surprised if the guy on the flight deck set the plane into descent and offed himself, but it sounds like the normal operation of the door would prevent this scenario.
The co-pilot joined Germanwings in September 2013, directly after training, and had flown 630 hours.
So, less than 18 months into the job.