Timeline cascade and fetal organ development
In your earlier question,
Why was Marty the last child to fade out?
the accepted answer posits:
It seems that the changes in the timeline cascade forward, affecting temporally displaced facts in order of how far removed they are from the moment of distortion. Marty faded last because his birthday is the furthest away.
Now consider this helpful information on fetal development from the Mayo Clinic. Organs begin to form at Week 5, in a particular order:
Fetal development three weeks after conception
The fifth week of pregnancy, or the third week after conception, marks the beginning of the embryonic period. This is when the baby's brain, spinal cord, heart and other organs begin to form.
Fetal development four weeks after conception
Growth is rapid this week. Just four weeks after conception, the neural tube along your baby's back is closing and your baby's heart is pumping blood.
Basic facial features will begin to appear, including passageways that will make up the inner ears and arches that will contribute to the jaw. Your baby's body begins to take on a C-shaped curvature. Small buds will soon become arms and legs.
Seven weeks into your pregnancy, or five weeks after conception, your baby's brain and face are rapidly developing. Tiny nostrils become visible, and the eye lenses begin to form. The arm buds that sprouted last week now take on the shape of paddles.
Applying the logic of the accepted answer for the question referenced above to Dave's head disappearing first, it could very well be that organs and body parts disappear in the order of "how far removed they are from the moment of distortion".
The heart is one of the first organs to appear, and so it may have actually disappeared before the head! The brain and arm buds / "paddle" hands form next, which could be why Dave's head disappears before the rest of his body and why Marty's hands start to disappear while he plays the guitar.