Here's a case where they're pretty sure it was all the vitamins and supplements in collagen capsules that the patient consumed for years https://n.neurology.org/content/98/18_Supplement/3654

No one has any idea. He wasn’t exposed to anything that would be high risk for it, if that’s even a thing. As another person commented here, it sounds like it’s spontaneous in many cases. Or at least that’s our best answer.