Failure of LEDs in the wet -- what's going on at the semiconductor level?
I think David's on the right track, but didn't explain thoroughly.  Dendritic growths and other impurities form a parallel path to the LED.  Now, these may not be linear, homogeneous paths...the impedance may change with time, temperature, humidity, etc.  One possibility is that the heat from the LED helps to complete the path, so that it brightens and dims.

If you have current available that will keep the LED on even when the alternate path is saturated, it will appear steady.  If the current can be easily absorbed by the other conductive path, it will likely flicker as the properties of that path change.