Went through the 'breakup' footage again. Noticed a few thing that I didn't see before.
It seems some of the left wing (actually, 'most the left wing' seems more like it) still stays intact despite the 'breakup' - I'm not sure, but to me it looks like something caused a big chunk of the trailing edge part of the left wing to fall of. The concentrated barrage of the .50s must have detached or sawed off something vital to keep that part together - but most parts of the left wing, are still left intact.
I went through the 'snaproll' part the 109 and noticed something I haven't seen before. The 'flipping over' might not have been entirely the result of the parts of the left wing falling off. Just as the shots begin to land, I've noticed what seems to be the aierlon deflected.
The first group of shots land near the left wingroot (and parts of the left rear fuselage). After that the barrage concentrates on the middle of the wing. The last, final big incendiary flash lands on the left wing, at the right edge of the left aielron and bam! The left aileron is suddenly deflected, however, the right aileron is not.
It could be something just hanging loose, instead of being an aileron, but if it is indeed an aileron, then the deflection wasn't caused by the pilot, since the right aileron stays neutral. There was no pilot input, the barrage of shots might have caused some kind of malfunction of the controls, the left aileron deflects as a big chunk of the left wing itself falls out, abrupt change in the attitude of the airflow over the left wing, and the 109 is 'snaprolled'. The speed of the 'roll', and attitude of it clearly indicates the plane being out of control, while the large part of the left wing is still intact.
So now..
I'm not sure if it was really the result of .50s "sawing off" and 'breaking up' the leftwing, that caused it to go out of control. It didn't 'saw it off' - but it clearly did something that wasn't good for maintaining control.