Sub blowing Christmas lights?

I assume the fuses are installed in fuse holders built into the plug of the light string.

The only thing I can think of is heat generated from poor contact pressure of the receptacle contacts against the plug contacts. Does the plug fit loose in the recept?

Al as a rule a fuse will not add protection, blow, in an over voltage situation. I could see bulbs burning out due to over voltage....

The bulbs I assume are a filament type purely a resistive load, I = E/R. I would agree the current will spike with the voltage increase but the fuse should not see the spike, jmho......

At any rate it seems "Sprink" has left the building. Without some further input from he or she we may never find the answer.
Jim