Less likely. Most aircraft crashes are caused by pilot error, and you'd have to postulate a failure that 1) caused the autopilot to fail and 2) simultaneously removed control from the pilot. This is more improbable than simple pilot error, thus Occam's Razor suggests that the simplest explanation (i.e. pilot error) is the correct one. Looking at the last dozen or so aircraft crashes, the Occam's Razor assumption has been right about 80% of the time. And if it was caused by 1) (the most common cause) you'd have a 0% chance of being right. Since that option is correct about 80% of the time, you in actuality have about a 20% chance of being right. The shielding in aircraft is actually better.