Joined: Jul 2015
Posts: 7,382
Likes: 3,226
From: NW Oregon
Bikes: 1982 Trek 930R Custom, '91 Diamondback Ascent w/ XT, XTR updates, Fuji Team Pro CF road flyer, Specialized Sirrus Gravel Convert, '09 Comencal Meta 5.5 XC, '02 Marin MBX500, '84 Gitane Criterium bike
there was probably a piece of dirt or sand grit in the valve when you last aired up the tube... re-airing it moved the grit out of the valve.
i always cap the tube valves to limit the dirt intrusion.
it can still get in.... sometimes from the pump side of things.