Originally Posted by gcl8a
That's not the question. Of course a nut can be made tighter.
But why does a wheel that doesn't slip with a derailer all of a sudden start slipping when made single-speed, all else being equal?
Because, faced with a choice, the girlyman side of your brain takes over and clicks the shifter.
Without a choice, your riding is much more agressive and you use a lot more body english, pedal kicks, yomping to keep the bike moving.
You also tend to run a lower gear with single speeds, as acceleration is more important than top speed. The smaller your front ring, and the larger your cog, the greater the leverage. Sprinting out of the saddle in the big ring puts much less force on your axle than grinding up a hill in granny gear.
Combine these factors and the quick release clamping force is overcome.