Although it may be that there is some health reason for the thinning I don't think it applies for her case. If you look at the braid, or the relative thickness, toward the head it is much thicker than farther down. This would put question about health reasons for thinning since it would happen from the scalp and be reflected all the way down.

Much more likely, the 'thinning' as it gets farther away from the head is from breakage. When she, or other models of very long hair, do 'hair flips, or subject their hair to the wind (or even tossing), some knots can occur. Knots are removed by combing or brushing and causes the hair to break off at that point. The volume form that point is now thinner.

Maybe we should be careful about what we ask of these ladies sometimes, in terms of hair flowing in the wind and such.