It's called "traction alopecia" and it could be the hipster's worst nightmare...pending there are no avocado shortages in the near future. A recent Mic.com article reported that man-buns could cause baldness via traction alopecia, in which the hair follicles are literally ripped out of the head. This is a result of prolonged tension and inflammation around the hair follicles, which can produce scarring and permanent hair loss.
However, there's no reason for guys everywhere to be freaking out. Mic reports that traction alopecia is mostly seen in the opposite sex: "...the documented condition of traction alopecia has generally been the purview of women, who in modern times have favored a larger variety of tight hairstyles." Still, "overly tight hair pulling, regardless of gender, could produce irreversible long-term consequences." Says the website—so, it's a possibility for those guys who tie their buns oppressively tight, but that whole situation can by avoided by simply loosening the damn bun. Or, ya know, just forgoing it altogether.