Theorists occasionally doubt it and skeptics demand double-blind tests. Practitioners laugh at them. Many have experienced it "first hand."
I hypothesised that the wood contracts and becomes "tighter" when it's left alone, and that the vibration of playing will open up the texture. She hypothesised that the glue will expand while playing, and when left alone will tighten up.
However, I've looked around a little and it's apparently a well-recognised phenomenon.
I would never have guessed, and found it interesting.
Compare and contrast:
http://www.google.co.uk/search?q=%22software+rots%22