The hair loss is because of the traumatic experience your body goes through during surgery. While under general anasthesia, your body basically goes into shock which can damage the hair follicles. I lost more hair after my knee surgery two years ago then I did after my sleeve. I have lost some hair after all of my surgeries where general anasthesia was used but the knee surgery was by far the most severe and I have no idea why.
Everyone is different. Some people who have never had surgery before WLS always blame the lack of Protein. I have proven this theory otherwise because my protein increased dramatically after surgery and I still had breakage (which is not loss). People who think they see the regrowth are actually seeing the hairs that were broken. I had breakage at different lenghts (most near the scalp) but it wasn't enough to make me freak out. No one could see the difference except me and the only difference I noticed was in the thickness. The breakage started around three months after all my surgeries and stopped in my 9th month post op (a 6 month period). I shed for about 15 months after knee surgery.
Everyone has their own experience though. Good luck to you and I wish you all the best (and little to zero hairloss).
“Systemic anesthesia has a profound effect on the body. As you would expect, it pushes the individual into a state of unconsciousness and relaxes muscles. Anesthesia also affects hair follicles. Hair follicles contain cells that are some of the fastest dividing and developing cells of the body. They have to divide fast to maintain the growth of hair Fiber. Anesthesia blocks this rapid cell division. While the application of anesthesia to an individual may only be for a few hours, the hair follicles may be affected to such a degree that they shut down hair fiber production and enter a telogen resting state. A telogen effluvium type of hair loss is the result.” {Source: Desai SP, Roaf ER. Telogen effluvium after anesthesia and surgery. Anesth Analg. 1984 Jan;63(1):83-4.}