Tires come in two flavors for this post, HIgh pressure (IE 100 PSI range) and low (Less than 50)
Low pressure tires on cars often have plastic camps on the tire valve stem, HIGH are supposed to have metal caps with a sealing ring, JUST in case the valve core does not seat properly.
My guess.. The valve stem was a tad loose (The nut at the base of the stem) and that is where it leaked.. This is, however, a guess, nothing more.
Suggestion: Dorian Pressure Pro or Pressure Pro or another Tire Pressure Monitoring System..
Ideally you should check your tire pressure quite often.. With many of these systems you can "Check" it from the driver's seat while doing 55mph every few minutes.. and if a tire goes low, IT screams at you "FEED ME AIR" like you would not believe.
The cost of such a system..... Less than your new tires.
