Air crash puzzle, page-65

These things are usually the result of a combination of factors if an accident.
NYT article
On the other hand if it's deliberate!!!

American experts say the typical procedure is for a flight attendant to use a food cart to block access to the cockpit when the pilot opens the door to leave. A flight attendant is supposed to remain in the cockpit and open the door for the pilot upon his or her return.
"It's a standard procedure" that Germanwings should have followed, Winn said. “That’s nothing secret. Everybody knows.”
If a member of the flight crew doesn't open the locked cockpit door from the inside, according to a manual for the Airbus A320 available online, the door can also be unlocked by the cabin crew outside the cockpit by entering a two- to seven-digit code, pre-programmed by the airline, on a keypad.

Door procedure.