They mention this in the article, but the physiology would suggest this is related to CSF/blood pooling in low G.
Taking it a step further, I bet this has a similar mechanism to IIH or the high pressure headaches you get with obstructive hydrocephalus. CSF is supposed to drain down via a relatively passive system. Without G to regulate this I can envision that you’d essentially develop the same physiology as someone with IIH (too much CSF).
Really interesting. A good example of how we have no idea what insane health things we are going to experience with space travel, but also how space travel may shed insight on treatments for other conditions with similar mechanisms we experience in a gravity well.