There are plenty of possible reasons.
From haircare related chemical damage to diet, excessive hair related heat exposure to health conditions.
Might be damaged hair as a result of bleaching or use of some very powerful haircare chemicals, such as developer at 30 and higher volume.
Could also be severely dehydrated hair.
This can occur as a result of overdrying, regularly washing hair in very hot water, damage from high temperature styling tools or failure to operate a haircare regime with effective deep care conditioning treatements/oils/hair masks/serums.
There may be some dietary deficiencies involved; particularly as regards a riboflavin, biotin, folate, and vitamin B12 deficiency.
Other possibilities could include: an endocrine disorder, hormonal imbalances connected with excessive levels of testesterone or androgens in the blood, side effects of prescribed meds or medium to long term contraceptives and stress.
If you have a history of pcos or thyroid problems in your family, make a GP appointment and get a blood test done as soon as possible.
Have you considered giving your hair a rinse in rice water a couple of times a month?
Maybe trying to incorporate coconut oil into your haircare regime?
Increasing your consumption of amino acids, seafood, seeds, raw olive oil and walnuts?