how do wheels become untrue you ask...mmm.. generally it starts when they are feeling neglected, maybe you haven't been for as many rides as you should, your riding over some rough patches.. but yes, wheels will become untrue when they start to lie to you, not coming home when they should.
They could be living it up with the Cervelo, the bianchi or even the Trek from the local bike club...but before you go and start a full on argument with your wheels, try and consider it from their perspective - sitting in the garage, often in the dark, for up to 20 hours a day, maybe, when your not too busy, they get to go out for a nice ride, but truthfully how often does that happen, when did you last buy your wheels some new tires, tubes, or even 2 dollar rim tape.
Go on, do sometihng for them now, a blow of fresh air, back up to the old 120psi. You'll be gald you did.