Had a hernia repair 8 mo ago, the incision just opened and drained grey/brown stuff out. what would cause it just to reopen out of the blue?

Call your surgeon: Unusual , call your surgeon , go for examination and will explain your problem and treat you.

Unsusual: Need to see your surgeon for an evaluation. Drainage 8 months after surgery would be unusual. An old hematoma may have drained. Was mesh used? Need to rule out a deep infection or mesh infection. Intestinal fistula would be rare.