The message
from Chris J Dixon contains these words:

The stirring causes the liquid level to rise at the edges and
thus increase the damping of the cup. Let it settle and it is
back as it was. Repeat at will.

Hmmm, in my experience it isn't repeatable. Only works with a freshly
poured cup, and restirring doesn't let it happen again. And anyway the
pitch continues to rise after rotation has ceased.

I've a feeling it's to do with microbubbles in the liquid that change
the speed of sound until they've all risen to the top. Perhaps I'll try
it with bicarb solution and add a drop of vinegar. If I can make it
repeat each time a drop's stirred in that may show the way.

--
Skipweasel
Pay no attention to that man behind the curtain.