When the sales start dropping; they know they are not going to release a new MacBook Air for about a year, so they can afford to let any returns sit in the warehouse for several months before pushing them out to boost sales.
It was defective. However, defects happen all the time, and usually they aren't resold until Apple makes the refurbished units more generally available.
Doubtful. Makes more sense to use refurbished systems to fulfill warranty-replacement needs than to use freshly built / new systems. Until Apple builds up whatever they consider an adequate "reserve", I doubt you'll see refurbs available for purchase.
I bought a 2011 13'' MBA last August when they were first released. I started watching the Apple refurb site in Oct and a few would pop up and be immediately purchased. Didn't see consistent large numbers of units for purchase until December. I finally bought a refurb 11'' in Jan.
It normally takes a while for them to be available. If you want an 8Gb model that may take even longer.