Don't trust "eyewitnesses" - stick them in front of a TV camera, or a reporter, and they say any old rubbish.
The one I listened to on the television seemed quite a serious sort, spoke clearly and didnt appear to be making anything but a factual statement of what he saw.
That was that the aircraft dropped a wing and then went into an almost vertical dive into the ground.
The wreckage would appear to be a high speed impact, the engine sound would indicate high speed too.
Why? is the bit that I would not like to speculate on because there could be numerous reasons from aircraft failure to pilot error, incapaciation etc.
All we do appear to know is the aircraft impacted at high speed and out of control. A very sad start to the year for us and an awful nightmare start to the year for for their family and friends.
Pace