The valve lost its ability to seal due to “vibration-induced relaxation” in threaded components that caused a mechanical failure in the valve, said John Horack, the Ohio State University professor who chaired the review board, at a briefing about the report.
“If you shake it sufficiently, you can get some changes in the mechanical configuration that will prevent the valve sealing. It’s pretty much no different than when your sink starts to drip,” he said.
As part of the investigation, engineers took a spare PCV and put it through shock and vibration environments like that experienced by the valve on the spacecraft. “We then put helium on it and after a moderate, small, number of cycles, the valve leaked. You could hear it leak from about four feet away,” he said. The report noted the leak rates in those tests were similar to those observed on the spacecraft.
Valves seem to be a recurring bane of spacecraft, don't they?