What do you mean by degrade? Did it go up or down?
I take it you were using a resistor and constant voltage supply rather than a constant current supply, if so there will be some feadback between the LED's voltage and the current.
Also, did the LEDs revert back to their normal voltage after the power had been disconnected and they were allowed to cool? I have a feeling that it's temperature related.
When I first saw this thread I thought you were talking about aging LEDs, for example the strike voltage of an arc lamp tends to increase as it ages, I wonder if the same is true for LEDs.