It can and will slip under completely dry conditions when those atmospheric conditions are just right.
It is caused by the very aim that they are trying to achieve of "no grease".
It would be interesting to see Mr Simpsons data if any was collected during the race.
Did the rise in revs occur on left hand bends by any chance.
The kart would have been quite undrivable at the point of slippage as the phenomenon would cause complete lack of drive.
The clutch causes metal fretting that is a rust dust and it accumulates in the drum.
while it is engaged it is not a problem, but if the cluch disengages, such as the tinkle you hear on left hand bends, the dust is released from its safe position in the corner of the drum.
This then settles on the shoe surface and acts like micro marbles between the drum and shoe when the clutch attempts to engage.
The frequency is beyond human hearing when it screeches and slips.
It can also cause the clutch to grab ferociously on pickup, snapping the shoe.
In addition, as it falls to the bottom when the kart is stationery and it appears the clutch shoe has failed as it drags the drum around due to the fine clearance.
A bit of grease hold the dust in suspension.
Full circle!
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