Strange happening with grey tank vent.

Just an educated guess, but something has pressurized that vent line. And it should not be pressurized ever. So something is, at least, partially blocking the vent. I'd do a cleanup of the gray tank, dawn+calgon soaked overnight followed by flushes. Repeat until you get semi-clear water out the drain. It's possible that there is something solid in the vent itself, a rag or something that went down a drain. In that case you might have to push that into the tank via the roof vent using a long rod or short 'snake'.

Of course someone will probably point out something obvious that I missed.