If it is even a possibility an airplane can stall, then at some point, there will be a stall. Inadvertant, of course, but you are in NO position to judge the other person. There can be a multitude of reasons. OBVIOUSLY IT WAS UN-INTENTIONAL. No one wakes up and says, yup, I'm gonna stall and crash this plane today! It was an un-desired state the aircraft ended up in, and then a botched recovery due largely to a POOR training program. Add to this LONG TIRING SCHEDULES, tiredness from a long day, fatigue, and you get a recipe in which an UN-DESIRED state can occurn in an aircraft (re: stall).
There is no single cause for an accident, but a bunch of links coming together to form the chain.