You'll never know. The design engineers may have overlooked a problem, maintenance crew may have broken something inadvertently, pilots may not have recognized a failed equipment (having a post it note over a rarely used warning light sounds funny, but is cited as one of the reasons for a famous nuclear power station failure) or maybe pilots could not agree on who would take over in time etc etc. The point is that there is so much redundancy in commercial aviation, one single mistake is usually not enough to bring a plane down, you need a series of them.