Usually by mis-using a frame pump. In this case, the wheel was locked to the frame when the bike fell over while I was loading my trailer (which causes the hitch side to get heavy). This must have caused the valve to get pinched against the lock. When I lifted the bike back up, I heard all the air come out and when I removed the lock, still wondering what happened, I saw half of my valve stem fly away from the bike. I walked the bike and loaded trailer to a place that sells inner tubes (sports store) about two blocks away because I had only 26" and 16" tubes with me. Plenty of patches!

Short answer, I did a lot of things wrong.