1) Catastrophic in-flight breakup due to foul play (e.g. bomb/missile): seems very unlikely such an event would exactly coincide with the change of heading.
2) Catastrophic in-flight breakup due to mid-air collision: possibly related to a change of heading but lack of any report of other aircraft being involved appears to rule this out.
3) Hijacking either by passengers or crew: the planned change of heading could have been used together with a disabling of the electronic systems to conceal a further re-routing.
4) Aircraft malfunction and/or pilot error: the change of heading could have been part of or have triggered a causal chain of events that brought down the airliner. Probably there's a few scenarios here such as, for example, the turn could have put forces on the repaired wing that caused it to fail.
It seems to me that scenarios 3 and, especially 4 are the most likely ones. Any other ideas what the coinciding change of heading and last contact might mean?
Re: 1) A bomb rigged with an accelerometer/compass trigger (essentially a smartphone would do it, programmed as some sort of inertial navigator) plotting the course of the aircraft - assuming the same flight files the same flight plan day in, day out - and then set to go off at this turn, because the perpetrator knows that at that point of the flight the AC will have reached cruising altitude and be somewhere over the sea?
A timer set to go off at XX o'clock would be a far more simple approach, agreed, if someone was plotting to bomb the AC. So just speculating.