I have also seen my own previously perfect neons get stained from a long storage together in a 10 gallon bin. I don't know for sure what caused it, but the temperature where they were stored was over 90 degrees Fahrenheit (I live in Texas), so I suspect it was the heat.
That is part of what made me want to do this was to see what exactly causes the fading. If they don't fade at room temperature (and various other temperatures), then that will help answer some questions.
MetalMonkey suggested that compression might be the cause, so I am willing to try that too. This might end up a long term project.
Edited by Behemoth, 05 August 2014 - 06:06 AM.