I think what happens is bubbles are formed in the fluid and it expands.
 This expansion is accommodated by the 16 psi pressure cap blow off valve

and

 some aerated fluid is forced into the overflow tank. Now we have aerated
fluid
 at the input to the pump allowing room for cavitation. Just a guess on my
part.

Yes, and also dont forget that the system is not incompressable.  The hoses
can expand, as can the radiator to a small degree.  Any air anywhere in the
system can also be compressed.  All that is plenty to allow for a few small
water vapor bubbles to form.

Dave Leonard

Good argument for wiggins couplings :-)

Paul Lamar

 
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