From Wikipedia:

Traction alopecia is a type of hair loss caused by a pulling force being applied to the hair.

This commonly results from a person frequently wearing their hair in a particularly tight ponytail, pigtails, or braids. It is also seen occasionally in long-haired people who use barrettes to keep hair out of their faces. Traction alopecia is recession of the hairline due to chronic traction, or hair pulling, and is characterized by a fringe along the marginal hairline on physical exam. Even though this "fringe sign" is considered a useful clinical marker of this condition, cases of frontal fibrosing alopecia presenting with an unusual retention of the hairline (pseudo-fringe sign) have been described.

Now, in our former Lock Shop, we never did see a case of alopecia which we knew for certain to be caused by dreadlocks. Against that, anyone concerned with this condition should balance the fact that dreadlocks retain all normally shed hairs, and thus fatten and become heavier over time - placing a small but possibly significant amount of extra weight on the gathered roots of each individual lock.

We have received and responded to many questions and concerns regarding whether its a good idea to have locks when a thinning hairline or baldness is present or a potential.

When facing thinning-related issues, from male-patterned baldness to simply fine, breakable hair, we recommend you consider carefully the fact that dreadlocks are heavy ropes of hair locked up with the thousands of hairs you normally would have shed and lost. This extra weight and added movement as they swing around is not going to do any favors to your already delicate hair-line.

If you already have locks and aren't ready to say goodbye to them, but want to minimize the damage, its a good idea to protect the roots with a strong, stretchy dreadband, hair wrap or hat to prevent excessive yanking and snapping of the hairs at the roots.

We also do not recommend skinning your locks back into a ponytail for this reason, as all that weight is now hanging off a few already-fragile roots around the hairline. For more information about dreadlock-related thinning and baldness, Google 'traction alopecia' where you will find a considerable amount of information about this hair-loss condition suffered by both women and men.